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Get ThemeResource textbox / richeditbox foreground and background color properties from code behind

Making a basic text editor with WinUI, MVVM. Stucked in default RichEditBox foreground and background color viewmodel properties based on selected theme, to show them in AppBar ColorPickerButtons.
Using Application.Current.Resources[]:
Tried different resource names and still not getting correct colors in dark theme (white foreground and whatever background color uses richeditbox). They always show light theme colors:
TextControlForegroundFocused (that TextBox Class msdn page
states it is "Text color when focused")
TextFillColorPrimary
SystemControlBackgroundAltHighBrush
SystemColorButtonFaceColor
SystemColorButtonTextColor
Same results using new TextBox().Resources as property value (TextBox because, again, msdn TextBox class page states that resources used by TextBox are shared with RichEditBox, among other controls. Same results using RichEditBox):
TextBoxForegroundThemeBrush
TextBoxBackgroundThemeBrush
So maybe the question is how to know what specific resource / themeresource names are used in specific control properties. I see resource names in resource xaml files are very descriptive but for now it is a mess for me. Also, I never see dark theme in those files, only Default, Light and HighContrast, and there are no colors matching dark theme, so it is evident that I am missing something (or many things :)

This is the default style of RichEditBox controls. You can get the ThemeResources's Keys.
<Style TargetType="RichEditBox">
<Setter Property="Foreground" Value="{ThemeResource TextControlForeground}" />
<Setter Property="Background" Value="{ThemeResource TextControlBackground}" />
<Setter Property="SelectionHighlightColor" Value="{ThemeResource TextControlSelectionHighlightColor}" />
<Setter Property="BorderBrush" Value="{ThemeResource TextControlBorderBrush}" />
<Setter Property="BorderThickness" Value="{ThemeResource TextControlBorderThemeThickness}" />
<Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}" />
<Setter Property="FontSize" Value="{ThemeResource ControlContentThemeFontSize}" />
<Setter Property="ScrollViewer.HorizontalScrollMode" Value="Auto" />
<Setter Property="ScrollViewer.VerticalScrollMode" Value="Auto" />
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto" />
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto" />
<Setter Property="ScrollViewer.IsDeferredScrollingEnabled" Value="False" />
<Setter Property="TextWrapping" Value="Wrap" />
<Setter Property="Padding" Value="{ThemeResource TextControlThemePadding}" />
<Setter Property="UseSystemFocusVisuals" Value="{ThemeResource IsApplicationFocusVisualKindReveal}" />
<Setter Property="ContextFlyout" Value="{StaticResource TextControlCommandBarContextFlyout}" />
<Setter Property="SelectionFlyout" Value="{StaticResource TextControlCommandBarSelectionFlyout}" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="RichEditBox">
<Grid>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="HeaderContentPresenter" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlHeaderForegroundDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBackgroundDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBorderBrushDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlForegroundDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="PlaceholderTextContentPresenter" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlPlaceholderForegroundDisabled}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Normal" />
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBorderBrushPointerOver}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBackgroundPointerOver}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="PlaceholderTextContentPresenter" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlPlaceholderForegroundPointerOver}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlForegroundPointerOver}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Focused">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="PlaceholderTextContentPresenter" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlPlaceholderForegroundFocused}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBackgroundFocused}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBorderBrushFocused}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlForegroundFocused}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement" Storyboard.TargetProperty="RequestedTheme">
<DiscreteObjectKeyFrame KeyTime="0" Value="Light" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="*" />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<ContentPresenter x:Name="HeaderContentPresenter"
Grid.Row="0"
Foreground="{ThemeResource TextControlHeaderForeground}"
Margin="{ThemeResource RichEditBoxTopHeaderMargin}"
Content="{TemplateBinding Header}"
ContentTemplate="{TemplateBinding HeaderTemplate}"
FontWeight="Normal"
TextWrapping="Wrap"
VerticalAlignment="Top"
Visibility="Collapsed"
x:DeferLoadStrategy="Lazy" />
<Border x:Name="BorderElement"
Grid.Row="1"
Grid.RowSpan="1"
Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
CornerRadius="{TemplateBinding CornerRadius}"
MinWidth="{ThemeResource TextControlThemeMinWidth}"
MinHeight="{ThemeResource TextControlThemeMinHeight}" />
<ScrollViewer x:Name="ContentElement"
Grid.Row="1"
HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}"
HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}"
VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}"
VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}"
IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}"
IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}"
IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}"
Margin="{TemplateBinding BorderThickness}"
Padding="{TemplateBinding Padding}"
IsTabStop="False"
ZoomMode="Disabled"
AutomationProperties.AccessibilityView="Raw" />
<TextBlock x:Name="PlaceholderTextContentPresenter"
Grid.Row="1"
Foreground="{ThemeResource TextControlPlaceholderForeground}"
Margin="{TemplateBinding BorderThickness}"
Padding="{TemplateBinding Padding}"
Text="{TemplateBinding PlaceholderText}"
TextAlignment="{TemplateBinding TextAlignment}"
TextWrapping="{TemplateBinding TextWrapping}"
IsHitTestVisible="False" />
<ContentPresenter x:Name="DescriptionPresenter"
Grid.Row="2"
Content="{TemplateBinding Description}"
Foreground="{ThemeResource SystemControlDescriptionTextForegroundBrush}"
AutomationProperties.AccessibilityView="Raw"
x:Load="False"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
For example, you can override the Background when the control is focused like this.
App.Current.Resources["TextControlBackgroundFocused"] = new SolidColorBrush(Colors.HotPink);

How to use a Custom Renderer for my Entry Controls

I am trying to change a Xamarin Forms (UWP) Entry Control's Vertical Centering based on this Answer.
I changed my App.xaml (In the UWP project) to:
<Application
x:Class="Prism.PointOfSale.UWP.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:Prism.PointOfSale.UWP"
RequestedTheme="Light">
<Application.Resources>
<Style x:Key="TextBoxStyle1" TargetType="TextBox">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="TextBox">
<Grid>
<Border x:Name="BorderElement" Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" Grid.ColumnSpan="2" Grid.RowSpan="1" Grid.Row="1"/>
<ContentPresenter x:Name="HeaderContentPresenter" VerticalAlignment="Center" ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" Grid.ColumnSpan="2" FontWeight="Normal" Foreground="{ThemeResource TextControlHeaderForeground}" Margin="0,0,0,8" Grid.Row="0" TextWrapping="{TemplateBinding TextWrapping}" Visibility="Collapsed" x:DeferLoadStrategy="Lazy"/>
<ScrollViewer x:Name="ContentElement" VerticalAlignment="Center" AutomationProperties.AccessibilityView="Raw" HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}" HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}" IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}" IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}" IsTabStop="False" IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}" Margin="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}" Grid.Row="1" VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}" VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}" ZoomMode="Disabled"/>
<TextBlock x:Name="PlaceholderTextContentPresenter" Grid.ColumnSpan="2" Foreground="{Binding PlaceholderForeground, RelativeSource={RelativeSource Mode=TemplatedParent}, TargetNullValue={ThemeResource TextControlPlaceholderForeground}}" IsHitTestVisible="False" Margin="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}" Grid.Row="1" Text="{TemplateBinding PlaceholderText}" TextWrapping="{TemplateBinding TextWrapping}" TextAlignment="{TemplateBinding TextAlignment}"/>
<Button x:Name="DeleteButton" AutomationProperties.AccessibilityView="Raw" BorderThickness="{TemplateBinding BorderThickness}" Grid.Column="1" FontSize="{TemplateBinding FontSize}" IsTabStop="False" MinWidth="34" Margin="{ThemeResource HelperButtonThemePadding}" Grid.Row="1" Style="{StaticResource DeleteButtonStyle}" VerticalAlignment="Stretch" Visibility="Collapsed"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Application.Resources>
</Application>
I added this Renderer to the UWP Project:
[assembly: ExportRenderer(typeof(Entry), typeof(CustomTextBoxRenderer))]
namespace Prism.PointOfSale.UWP
{
public class CustomTextBoxRenderer : EntryRenderer
{
protected override void OnElementChanged(ElementChangedEventArgs<Entry> e)
{
base.OnElementChanged(e);
if (this.Control != null)
{
this.Control.Height = 60;
Control.Style = (Windows.UI.Xaml.Style)App.Current.Resources["TextBoxStyle1"];
}
}
}
}
The Build locked up for about 2 minutes. But, my text is still on the bottom of the Entry Control after the form loaded. What else do I need to do to use the custom renderer?

The Build locked up for about 2 minutes. But, my text is still on the bottom of the Entry Control after the form loaded.
The problem is your TextBoxStyle1 is not complete, please copy the following style to replace, and set ContentElement scrollviewer VerticalAlignment as Center.
<Style x:Key="TextBoxStyle1" TargetType="TextBox">
<Setter Property="Foreground" Value="{ThemeResource TextControlForeground}" />
<Setter Property="Background" Value="{ThemeResource TextControlBackground}" />
<Setter Property="BorderBrush" Value="{ThemeResource TextControlBorderBrush}" />
<Setter Property="SelectionHighlightColor" Value="{ThemeResource TextControlSelectionHighlightColor}" />
<Setter Property="BorderThickness" Value="{ThemeResource TextControlBorderThemeThickness}" />
<Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}" />
<Setter Property="FontSize" Value="{ThemeResource ControlContentThemeFontSize}" />
<Setter Property="ScrollViewer.HorizontalScrollMode" Value="Auto" />
<Setter Property="ScrollViewer.VerticalScrollMode" Value="Auto" />
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Hidden" />
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Hidden" />
<Setter Property="ScrollViewer.IsDeferredScrollingEnabled" Value="False" />
<Setter Property="Padding" Value="{ThemeResource TextControlThemePadding}" />
<Setter Property="UseSystemFocusVisuals" Value="{ThemeResource IsApplicationFocusVisualKindReveal}" />
<Setter Property="ContextFlyout" Value="{StaticResource TextControlCommandBarContextFlyout}" />
<Setter Property="SelectionFlyout" Value="{StaticResource TextControlCommandBarSelectionFlyout}" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="TextBox">
<Grid>
<Grid.Resources>
<Style x:Name="DeleteButtonStyle" TargetType="Button">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Button">
<Grid
x:Name="ButtonLayoutGrid"
Background="{ThemeResource TextControlButtonBackground}"
BorderBrush="{ThemeResource TextControlButtonBorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}">
<TextBlock
x:Name="GlyphElement"
HorizontalAlignment="Center"
VerticalAlignment="Center"
AutomationProperties.AccessibilityView="Raw"
FontFamily="{ThemeResource SymbolThemeFontFamily}"
FontSize="12"
FontStyle="Normal"
Foreground="{ThemeResource TextControlButtonForeground}"
Text="" />
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal" />
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ButtonLayoutGrid" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlButtonBackgroundPointerOver}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ButtonLayoutGrid" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlButtonBorderBrushPointerOver}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="GlyphElement" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlButtonForegroundPointerOver}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ButtonLayoutGrid" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlButtonBackgroundPressed}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ButtonLayoutGrid" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlButtonBorderBrushPressed}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="GlyphElement" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlButtonForegroundPressed}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<DoubleAnimation
Storyboard.TargetName="ButtonLayoutGrid"
Storyboard.TargetProperty="Opacity"
To="0"
Duration="0" />
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Grid.Resources>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="*" />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>
<ContentPresenter
x:Name="HeaderContentPresenter"
Grid.Row="0"
Grid.Column="0"
Grid.ColumnSpan="2"
Margin="{ThemeResource TextBoxTopHeaderMargin}"
VerticalAlignment="Top"
x:DeferLoadStrategy="Lazy"
Content="{TemplateBinding Header}"
ContentTemplate="{TemplateBinding HeaderTemplate}"
FontWeight="Normal"
Foreground="{ThemeResource TextControlHeaderForeground}"
TextWrapping="Wrap"
Visibility="Collapsed" />
<Border
x:Name="BorderElement"
Grid.Row="1"
Grid.RowSpan="1"
Grid.Column="0"
Grid.ColumnSpan="2"
MinWidth="{ThemeResource TextControlThemeMinWidth}"
MinHeight="{ThemeResource TextControlThemeMinHeight}"
Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Control.IsTemplateFocusTarget="True"
CornerRadius="{TemplateBinding CornerRadius}" />
<ScrollViewer
x:Name="ContentElement"
Grid.Row="1"
Grid.Column="0"
Margin="{TemplateBinding BorderThickness}"
Padding="{TemplateBinding Padding}"
VerticalAlignment="Center"
AutomationProperties.AccessibilityView="Raw"
HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}"
HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}"
IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}"
IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}"
IsTabStop="False"
IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}"
VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}"
VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}"
ZoomMode="Disabled" />
<TextBlock
x:Name="PlaceholderTextContentPresenter"
Grid.Row="1"
Grid.Column="0"
Grid.ColumnSpan="2"
Margin="{TemplateBinding BorderThickness}"
Padding="{TemplateBinding Padding}"
Foreground="{Binding PlaceholderForeground, RelativeSource={RelativeSource TemplatedParent}, TargetNullValue={ThemeResource TextControlPlaceholderForeground}}"
IsHitTestVisible="False"
Text="{TemplateBinding PlaceholderText}"
TextAlignment="{TemplateBinding TextAlignment}"
TextWrapping="{TemplateBinding TextWrapping}" />
<Button
x:Name="DeleteButton"
Grid.Row="1"
Grid.Column="1"
MinWidth="34"
Margin="{ThemeResource HelperButtonThemePadding}"
VerticalAlignment="Stretch"
AutomationProperties.AccessibilityView="Raw"
BorderThickness="{TemplateBinding BorderThickness}"
FontSize="{TemplateBinding FontSize}"
IsTabStop="False"
Style="{StaticResource DeleteButtonStyle}"
Visibility="Collapsed" />
<ContentPresenter
x:Name="DescriptionPresenter"
Grid.Row="2"
Grid.Column="0"
Grid.ColumnSpan="2"
x:Load="False"
AutomationProperties.AccessibilityView="Raw"
Content="{TemplateBinding Description}"
Foreground="{ThemeResource SystemControlDescriptionTextForegroundBrush}" />
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal" />
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="HeaderContentPresenter" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlHeaderForegroundDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBackgroundDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBorderBrushDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlForegroundDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="PlaceholderTextContentPresenter" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{Binding PlaceholderForeground, RelativeSource={RelativeSource TemplatedParent}, TargetNullValue={ThemeResource TextControlPlaceholderForegroundDisabled}}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBorderBrushPointerOver}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBackgroundPointerOver}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="PlaceholderTextContentPresenter" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{Binding PlaceholderForeground, RelativeSource={RelativeSource TemplatedParent}, TargetNullValue={ThemeResource TextControlPlaceholderForegroundPointerOver}}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlForegroundPointerOver}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Focused">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="PlaceholderTextContentPresenter" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{Binding PlaceholderForeground, RelativeSource={RelativeSource TemplatedParent}, TargetNullValue={ThemeResource TextControlPlaceholderForegroundFocused}}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBackgroundFocused}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBorderBrushFocused}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlForegroundFocused}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement" Storyboard.TargetProperty="RequestedTheme">
<DiscreteObjectKeyFrame KeyTime="0" Value="Light" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="ButtonStates">
<VisualState x:Name="ButtonVisible">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="DeleteButton" Storyboard.TargetProperty="Visibility">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="ButtonCollapsed" />
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

How To increase the calendar icon size of CalendarDatePicker and customising it with different text or image icon [UWP]?

I tried changing the calendar icon size but couldn't find a property to achieve this.

How To increase the calendar icon size of CalendarDatePicker and customising it with different text or image icon [UWP]?
You could edit the default CalendarDatePicker style and change CalendarGlyph FontIcon's FontSize and Glyph property like the following.
<Style TargetType="CalendarDatePicker">
<Setter Property="Foreground" Value="{ThemeResource CalendarDatePickerForeground}" />
<Setter Property="Background" Value="{ThemeResource CalendarDatePickerBackground}" />
<Setter Property="BorderBrush" Value="{ThemeResource CalendarDatePickerBorderBrush}" />
<Setter Property="BorderThickness" Value="{ThemeResource CalendarDatePickerBorderThemeThickness}" />
<Setter Property="HorizontalAlignment" Value="Left" />
<Setter Property="VerticalAlignment" Value="Center" />
<Setter Property="UseSystemFocusVisuals" Value="{ThemeResource IsApplicationFocusVisualKindReveal}" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="CalendarDatePicker">
<Grid x:Name="Root">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal" />
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Background" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerBorderBrushPointerOver}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Background" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerBackgroundPointerOver}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Background" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerBackgroundPressed}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Background" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerBorderBrushPressed}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Background" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerBackgroundDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Background" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerBorderBrushDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="HeaderContentPresenter" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerHeaderForegroundDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="DateText" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerTextForegroundDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="CalendarGlyph" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerCalendarGlyphForegroundDisabled}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="FocusStates">
<VisualState x:Name="Unfocused" />
<VisualState x:Name="PointerFocused" />
<VisualState x:Name="Focused">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Background" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerBackgroundFocused}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="SelectionStates">
<VisualState x:Name="Unselected" />
<VisualState x:Name="Selected">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="DateText" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerTextForegroundSelected}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="HeaderStates">
<VisualState x:Name="TopHeader" />
<VisualState x:Name="LeftHeader">
<VisualState.Setters>
<Setter Target="HeaderContentPresenter.(Grid.Row)" Value="1" />
<Setter Target="HeaderContentPresenter.(Grid.Column)" Value="0" />
<Setter Target="HeaderContentPresenter.(Grid.ColumnSpan)" Value="1" />
<Setter Target="HeaderContentPresenter.Margin" Value="{StaticResource CalendarDatePickerLeftHeaderMargin}" />
<Setter Target="HeaderContentPresenter.MaxWidth" Value="{StaticResource CalendarDatePickerLeftHeaderMaxWidth}" />
</VisualState.Setters>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<FlyoutBase.AttachedFlyout>
<Flyout Placement="Bottom" ShouldConstrainToRootBounds="False">
<Flyout.FlyoutPresenterStyle>
<Style TargetType="FlyoutPresenter">
<Setter Property="Padding" Value="0" />
<Setter Property="BorderThickness" Value="0" />
<Setter Property="IsDefaultShadowEnabled" Value="True" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="FlyoutPresenter">
<ContentPresenter Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Content="{TemplateBinding Content}"
ContentTemplate="{TemplateBinding ContentTemplate}"
ContentTransitions="{TemplateBinding ContentTransitions}"
Margin="{TemplateBinding Padding}"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Flyout.FlyoutPresenterStyle>
<CalendarView x:Name="CalendarView"
Style="{TemplateBinding CalendarViewStyle}"
MinDate="{TemplateBinding MinDate}"
MaxDate="{TemplateBinding MaxDate}"
IsTodayHighlighted="{TemplateBinding IsTodayHighlighted}"
DisplayMode="{TemplateBinding DisplayMode}"
FirstDayOfWeek="{TemplateBinding FirstDayOfWeek}"
DayOfWeekFormat="{TemplateBinding DayOfWeekFormat}"
CalendarIdentifier="{TemplateBinding CalendarIdentifier}"
IsOutOfScopeEnabled="{TemplateBinding IsOutOfScopeEnabled}"
IsGroupLabelVisible="{TemplateBinding IsGroupLabelVisible}" />
</Flyout>
</FlyoutBase.AttachedFlyout>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="*" />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="*" />
<ColumnDefinition Width="32" />
</Grid.ColumnDefinitions>
<ContentPresenter x:Name="HeaderContentPresenter"
x:DeferLoadStrategy="Lazy"
Grid.Row="0"
Grid.Column="1"
Grid.ColumnSpan="2"
Content="{TemplateBinding Header}"
ContentTemplate="{TemplateBinding HeaderTemplate}"
Margin="{StaticResource CalendarDatePickerTopHeaderMargin}"
TextWrapping="Wrap"
VerticalAlignment="Top"
Visibility="Collapsed" />
<Border x:Name="Background"
Grid.Row="1"
Grid.Column="1"
Grid.ColumnSpan="2"
BorderThickness="{TemplateBinding BorderThickness}"
BorderBrush="{TemplateBinding BorderBrush}"
Background="{TemplateBinding Background}"
CornerRadius="{TemplateBinding CornerRadius}"
Control.IsTemplateFocusTarget="True"
MinHeight="32"/>
<TextBlock x:Name="DateText"
Grid.Row="1"
Grid.Column="1"
HorizontalAlignment="Left"
Foreground="{ThemeResource CalendarDatePickerTextForeground}"
Text="{TemplateBinding PlaceholderText}"
Padding="12, 0, 0, 2"
VerticalAlignment="Center" />
<!--Edit the following FontIcon's property-->
<FontIcon x:Name="CalendarGlyph"
Grid.Row="1"
Grid.Column="2"
Glyph=""
FontFamily="{ThemeResource SymbolThemeFontFamily}"
Foreground="{ThemeResource CalendarDatePickerCalendarGlyphForeground}"
FontSize="18"
HorizontalAlignment="Center"
VerticalAlignment="Center" />
<ContentPresenter x:Name="DescriptionPresenter"
Grid.Row="2"
Grid.Column="1"
Grid.ColumnSpan="2"
Content="{TemplateBinding Description}"
x:Load="False"
Foreground="{ThemeResource SystemControlDescriptionTextForegroundBrush}"
AutomationProperties.AccessibilityView="Raw" />
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

How to increase the height of the UWP calender date picker?

I have added UWP calendar date picker. Here is the XAML code
<CalendarDatePicker Background="White" Grid.Row="6" Width="660" VerticalAlignment="Stretch" FontSize="48" PlaceholderText="Date of Birth" Padding="0" Margin="0" x:Name="DobDatePicker"></CalendarDatePicker>
But the problem is that I can't change the height of that calendar textblock control(not the pop up). I tried by setting the minimum height, but it does n't work. Is there any way to fix this issue?

CalenderDatePicker has hardcoded value(32) for its rowdefinition in default style. So, You can modify that particular part of the CalenderDatePicker default style to change the height.
//Portion to modified
<Style TargetType="CalendarDatePicker">
....
....
<Grid x:Name="Root">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="32" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="32" /> /* change your value */
</Grid.RowDefinitions>
....
....
</Style>
Screenshot for your reference
Complete Style for your reference
//C# Code
<CalendarDatePicker>
<CalendarDatePicker.Style>
<Style TargetType="CalendarDatePicker">
<Setter Property="Foreground" Value="{ThemeResource CalendarDatePickerForeground}" />
<Setter Property="Background" Value="{ThemeResource CalendarDatePickerBackground}" />
<Setter Property="BorderBrush" Value="{ThemeResource CalendarDatePickerBorderBrush}" />
<Setter Property="BorderThickness" Value="{ThemeResource CalendarDatePickerBorderThemeThickness}" />
<Setter Property="HorizontalAlignment" Value="Left" />
<Setter Property="VerticalAlignment" Value="Center" />
<Setter Property="UseSystemFocusVisuals" Value="{ThemeResource IsApplicationFocusVisualKindReveal}" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="CalendarDatePicker">
<Grid x:Name="Root">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="32" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="32" />
</Grid.RowDefinitions>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal" />
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Background" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerBorderBrushPointerOver}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Background" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerBackgroundPointerOver}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Background" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerBackgroundPressed}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Background" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerBorderBrushPressed}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Background" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerBackgroundDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Background" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerBorderBrushDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="HeaderContentPresenter" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerHeaderForegroundDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="DateText" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerTextForegroundDisabled}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="CalendarGlyph" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerCalendarGlyphForegroundDisabled}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="FocusStates">
<VisualState x:Name="Unfocused" />
<VisualState x:Name="PointerFocused" />
<VisualState x:Name="Focused">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Background" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerBackgroundFocused}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="SelectionStates">
<VisualState x:Name="Unselected" />
<VisualState x:Name="Selected">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="DateText" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource CalendarDatePickerTextForegroundSelected}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<FlyoutBase.AttachedFlyout>
<Flyout Placement="Bottom">
<Flyout.FlyoutPresenterStyle>
<Style TargetType="FlyoutPresenter">
<Setter Property="Padding" Value="0" />
<Setter Property="BorderThickness" Value="0" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="FlyoutPresenter">
<ContentPresenter Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Content="{TemplateBinding Content}"
ContentTemplate="{TemplateBinding ContentTemplate}"
ContentTransitions="{TemplateBinding ContentTransitions}"
Margin="{TemplateBinding Padding}"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Flyout.FlyoutPresenterStyle>
<CalendarView x:Name="CalendarView"
Style="{TemplateBinding CalendarViewStyle}"
MinDate="{TemplateBinding MinDate}"
MaxDate="{TemplateBinding MaxDate}"
IsTodayHighlighted="{TemplateBinding IsTodayHighlighted}"
DisplayMode="{TemplateBinding DisplayMode}"
FirstDayOfWeek="{TemplateBinding FirstDayOfWeek}"
DayOfWeekFormat="{TemplateBinding DayOfWeekFormat}"
CalendarIdentifier="{TemplateBinding CalendarIdentifier}"
IsOutOfScopeEnabled="{TemplateBinding IsOutOfScopeEnabled}"
IsGroupLabelVisible="{TemplateBinding IsGroupLabelVisible}" />
</Flyout>
</FlyoutBase.AttachedFlyout>
<ContentPresenter x:Name="HeaderContentPresenter"
x:DeferLoadStrategy="Lazy"
Margin="{ThemeResource ComboBoxHeaderThemeMargin}"
Visibility="Collapsed"
Content="{TemplateBinding Header}"
ContentTemplate="{TemplateBinding HeaderTemplate}"
TextWrapping="Wrap"/>
<Border x:Name="Background"
Grid.Row="1"
Grid.ColumnSpan="2"
BorderThickness="{TemplateBinding BorderThickness}"
BorderBrush="{TemplateBinding BorderBrush}"
Background="{TemplateBinding Background}"
Control.IsTemplateFocusTarget="True" />
<TextBlock x:Name="DateText"
HorizontalAlignment="Left"
Foreground="{ThemeResource CalendarDatePickerTextForeground}"
Text="{TemplateBinding PlaceholderText}"
Grid.Row="1"
FontSize="15"
Padding="12, 0, 0, 2"
VerticalAlignment="Center" />
<FontIcon x:Name="CalendarGlyph"
Glyph=""
FontFamily="{ThemeResource SymbolThemeFontFamily}"
Foreground="{ThemeResource CalendarDatePickerCalendarGlyphForeground}"
Grid.Row="1"
Grid.Column="1"
FontSize="16"
HorizontalAlignment="Center"
VerticalAlignment="Center" />
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</CalendarDatePicker.Style>
</CalendarDatePicker>

Override ThemeResource for specific controls

I want to create a transparent TextBox, with no background color when focused (default is white, or SystemControlBackgroundChromeWhiteBrush).
I understand that I can override the used color in the App.xaml like so:
<Application.Resources>
<SolidColorBrush x:Key="SystemControlBackgroundChromeWhiteBrush">Transparent</SolidColorBrush>
</Application.Resources>
(or by using multiple ThemeDictionaries)
However I don't want to modify ALL TextBlocks but only specific ones. My first guess was to include the changed brush in the TextBlock's Resources, but this doesn't work.

You can modify default styles of control. HERE is the article how to find it.
Get style of TextBox, insert it as a resource in XAML and modify, eg.:
<!-- Default style for Windows.UI.Xaml.Controls.TextBox -->
<Style TargetType="TextBox">
<Setter Property="MinWidth" Value="{ThemeResource TextControlThemeMinWidth}" />
<Setter Property="MinHeight" Value="{ThemeResource TextControlThemeMinHeight}" />
<Setter Property="Foreground" Value="{ThemeResource TextBoxForegroundThemeBrush}" />
<Setter Property="SelectionHighlightColor" Value="{ThemeResource TextSelectionHighlightColorThemeBrush}" />
<Setter Property="Background" Value="{ThemeResource TextBoxBackgroundThemeBrush}" />
<Setter Property="BorderBrush" Value="{ThemeResource TextBoxBorderThemeBrush}" />
<Setter Property="BorderThickness" Value="{ThemeResource TextControlBorderThemeThickness}" />
<Setter Property="FontFamily" Value="{ThemeResource PhoneFontFamilyNormal}" />
<Setter Property="FontSize" Value="{ThemeResource ContentControlFontSize}" />
<Setter Property="TextWrapping" Value="NoWrap" />
<Setter Property="ScrollViewer.HorizontalScrollMode" Value="Auto" />
<Setter Property="ScrollViewer.VerticalScrollMode" Value="Auto" />
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Hidden" />
<Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Hidden" />
<Setter Property="ScrollViewer.IsDeferredScrollingEnabled" Value="False" />
<Setter Property="Padding" Value="{ThemeResource TextControlThemePadding}" />
<Setter Property="Margin" Value="{ThemeResource TextControlMarginThemeThickness}" />
<Setter Property="VerticalAlignment" Value="Top" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="TextBox">
<Grid Background="Transparent">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxDisabledBackgroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxDisabledBorderThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxDisabledForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="PlaceholderTextContentPresenter" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxDisabledForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="HeaderContentPresenter" Storyboard.TargetProperty="Foreground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxDisabledHeaderForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Normal">
<Storyboard>
<DoubleAnimation Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="Opacity" Duration="0"
To="{ThemeResource TextControlBorderThemeOpacity}" />
</Storyboard>
</VisualState>
<VisualState x:Name="Focused">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextSelectionHighlightColorThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<DoubleAnimation Storyboard.TargetName="PlaceholderTextContentPresenter" Storyboard.TargetProperty="Opacity" Duration="0" To="0" />
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxFocusedBackgroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<Border x:Name="BorderElement" Grid.Row="1" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" />
<ContentPresenter x:Name="HeaderContentPresenter" Grid.Row="0" Style="{StaticResource HeaderContentPresenterStyle}" Margin="{ThemeResource TextControlHeaderMarginThemeThickness}"
Content="{TemplateBinding Header}" ContentTemplate="{TemplateBinding HeaderTemplate}" />
<ScrollViewer x:Name="ContentElement" Grid.Row="1" MinHeight="{ThemeResource TextControlThemeMinHeight}"
HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}"
HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}"
VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}"
VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}"
IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}"
IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}"
IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}" Margin="{TemplateBinding BorderThickness}"
Padding="{TemplateBinding Padding}" IsTabStop="False" ZoomMode="Disabled"
AutomationProperties.AccessibilityView="Raw"/>
<ContentControl x:Name="PlaceholderTextContentPresenter" Grid.Row="1" Foreground="{ThemeResource TextBoxPlaceholderTextThemeBrush}"
FontSize="{ThemeResource ContentControlFontSize}" Margin="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}"
IsTabStop="False" Content="{TemplateBinding PlaceholderText}" />
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
Next, add name to modified style:
<Style x:Name="CustomTextBlock" TargetType="TextBox">
And apply it to your control, eg:
<TextBlock Style="{StaticResource CustomTextBlock}"/>
Here is the sample style for Transparent TextBox (WinRT/Universal Apps):
<Style x:Key="TransparentTextBox"
TargetType="TextBox">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="TextBox">
<Grid Background="Transparent">
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush"
Storyboard.TargetName="BorderElement">
<DiscreteObjectKeyFrame KeyTime="0"
Value="{ThemeResource TextBoxDisabledBorderThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground"
Storyboard.TargetName="ContentElement">
<DiscreteObjectKeyFrame KeyTime="0"
Value="{ThemeResource TextBoxDisabledForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground"
Storyboard.TargetName="PlaceholderTextContentPresenter">
<DiscreteObjectKeyFrame KeyTime="0"
Value="{ThemeResource TextBoxDisabledForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground"
Storyboard.TargetName="HeaderContentPresenter">
<DiscreteObjectKeyFrame KeyTime="0"
Value="{ThemeResource TextBoxDisabledHeaderForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Normal">
<Storyboard>
<DoubleAnimation Duration="0"
To="{ThemeResource TextControlBorderThemeOpacity}"
Storyboard.TargetProperty="Opacity"
Storyboard.TargetName="BorderElement" />
</Storyboard>
</VisualState>
<VisualState x:Name="Focused">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush"
Storyboard.TargetName="BorderElement">
<DiscreteObjectKeyFrame KeyTime="0"
Value="{ThemeResource TextSelectionHighlightColorThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<DoubleAnimation Duration="0"
To="0"
Storyboard.TargetProperty="Opacity"
Storyboard.TargetName="PlaceholderTextContentPresenter" />
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border x:Name="BorderElement"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Grid.Row="1" />
<ContentPresenter x:Name="HeaderContentPresenter"
ContentTemplate="{TemplateBinding HeaderTemplate}"
Content="{TemplateBinding Header}"
Margin="{ThemeResource TextControlHeaderMarginThemeThickness}"
Grid.Row="0"
Style="{StaticResource HeaderContentPresenterStyle}" />
<ScrollViewer x:Name="ContentElement"
AutomationProperties.AccessibilityView="Raw"
HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}"
HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}"
IsTabStop="False"
IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}"
IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}"
IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}"
Margin="{TemplateBinding BorderThickness}"
MinHeight="{ThemeResource TextControlThemeMinHeight}"
Padding="{TemplateBinding Padding}"
Grid.Row="1"
VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}"
VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}"
ZoomMode="Disabled" />
<ContentControl x:Name="PlaceholderTextContentPresenter"
Content="{TemplateBinding PlaceholderText}"
Foreground="{ThemeResource TextBoxPlaceholderTextThemeBrush}"
FontSize="{ThemeResource ContentControlFontSize}"
IsTabStop="False"
Margin="{TemplateBinding BorderThickness}"
Padding="{TemplateBinding Padding}"
Grid.Row="1" />
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Furthermore, you can use Blend (tool installed with Visual Studio), to modify controls easier. HERE is the sample tutorial from Microsoft Virtual Academy how to use it with VS2013.

You can use styles much like CSS and apply them only to specific controls:
<Style x:Key="TransparentTextBox" TargetType="{x:Type TextBox}">
<Setter Property="Background" Value="Transparent" />
</Style>
And then on the control reference the style you want to use:
<TextBox Style="{StaticResource TransparentTextBox}" />
Hope this helps.