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Why is my ListView-Item only jumping back to VisualState Selected, once?

I have a problem with my ListView in my UWP-Application. When selecting an Item the first time, after MouseOver ends it's VisualState changes to "Selected" as it should, but when I select it a second time, after I selected another Item, it stayes in VisualState "Pressed" even after MouseOver until I select the next Item. So my Item switches only once to the desired VisualState, to iggnore this State for the rest of the session.
What could be the reason for this behaviour, and could this be unique to UWP, since I do not remember having seen something like this before.
This is my ListView, as I use it in Xaml right now:
<ListView IsItemClickEnabled="True"
Grid.Row="1"
Grid.Column="1"
ScrollViewer.VerticalScrollMode="Auto"
ScrollViewer.VerticalScrollBarVisibility="Auto"
ItemsSource="{Binding Path=list, Mode=OneWay}"
ItemTemplate="{StaticResource RepVSmallIcon70ItemTemplate}"
TabIndex="1"
SelectedIndex="{Binding Path=selectedIndex, Mode=TwoWay}"
SelectionChanged="ListView_SelectionChanged"
IsRightTapEnabled="False"
ItemContainerStyle="{StaticResource ListViewItemStyleUWP}"/>
which uses this ItemContainerStyle:
<Style x:Key="ListViewItemStyleUWP" TargetType="ListViewItem">
<Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}"/>
<Setter Property="FontSize" Value="{ThemeResource ControlContentThemeFontSize}"/>
<Setter Property="Background" Value="MediumOrchid"/>
<Setter Property="Foreground" Value="{ThemeResource SystemControlForegroundBaseHighBrush}"/>
<Setter Property="TabNavigation" Value="Local"/>
<Setter Property="IsHoldingEnabled" Value="True"/>
<Setter Property="Padding" Value="12,0,12,0"/>
<Setter Property="HorizontalContentAlignment" Value="Left"/>
<Setter Property="VerticalContentAlignment" Value="Center"/>
<Setter Property="MinWidth" Value="160"/>
<Setter Property="MinHeight" Value="100"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ListViewItem">
<Border x:Name="OuterContainer" RenderTransformOrigin="0.5,0.5">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal">
<!-- unselektiert -->
<Storyboard>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="Orange"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="LightBlue"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="Margin">
<DiscreteObjectKeyFrame KeyTime="0" Value="0 0 0 2"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<!-- Klick -->
<Storyboard>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="Red"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="Blue"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="Margin">
<DiscreteObjectKeyFrame KeyTime="0" Value="2 2 2 4"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="SelectionStates">
<VisualState x:Name="Selected">
<!--Selektiert-->
<Storyboard>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="DarkRed"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="DarkBlue"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="Margin">
<DiscreteObjectKeyFrame KeyTime="0" Value="0 0 0 2"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Grid x:Name="ReorderHintContent" Background="Transparent">
<Border x:Name="ContentContainer">
<Border x:Name="ContentBorder"
Background="LightCyan"
BorderBrush="DarkCyan"
BorderThickness="2">
<Grid>
<ContentPresenter x:Name="contentPresenter"
ContentTransitions="{TemplateBinding ContentTransitions}"
ContentTemplate="{TemplateBinding ContentTemplate}"
Content="{TemplateBinding Content}"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
Margin="{TemplateBinding Padding}" />
</Grid>
</Border>
</Border>
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

The ListViewItem styles and templates doesn't have SelectionStates VisualStateGroups. So the ListViewItem style doesn't manage your selected view state in the SelectionStates well. Move the Selected VisualState to the CommonStates VisualStateGroup. And it will work.
Updated code as follows:
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ListViewItem">
<Border x:Name="OuterContainer" RenderTransformOrigin="0.5,0.5">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal">
<!-- unselektiert -->
<Storyboard>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="Orange" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="LightBlue" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="Margin">
<DiscreteObjectKeyFrame KeyTime="0" Value="0 0 0 2" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<!-- Klick -->
<Storyboard>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="Red" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="Blue" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="Margin">
<DiscreteObjectKeyFrame KeyTime="0" Value="2 2 2 4" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Selected">
<!-- Selektiert -->
<Storyboard>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="BorderBrush">
<DiscreteObjectKeyFrame KeyTime="0" Value="DarkRed" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="Background">
<DiscreteObjectKeyFrame KeyTime="0" Value="DarkBlue" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Duration="0"
Storyboard.TargetName="ContentBorder"
Storyboard.TargetProperty="Margin">
<DiscreteObjectKeyFrame KeyTime="0" Value="0 0 0 2" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Grid x:Name="ReorderHintContent" Background="Transparent">
<Border x:Name="ContentContainer">
<Border x:Name="ContentBorder"
Background="LightCyan"
BorderBrush="DarkCyan"
BorderThickness="2">
<Grid>
<ContentPresenter x:Name="contentPresenter"
Margin="{TemplateBinding Padding}"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
Content="{TemplateBinding Content}"
ContentTemplate="{TemplateBinding ContentTemplate}"
ContentTransitions="{TemplateBinding ContentTransitions}" />
</Grid>
</Border>
</Border>
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>

Center placeholder text in TextBox

I have multiple TextBoxes in my XAML-Page.
<TextBox Name="TxtCompanyId" PlaceholderText="FirmaId" Height="40" Margin="5" TextAlignment="Center" />
The TextAlignment="Center" property only centers the text of the TextBox and not the placeholdertext/hint text. Is there a way to center both?

You can surely modify the defalut style and set PlaceholderTextContentPresenter's horizontal alignment to Center:
<ContentControl x:Name="PlaceholderTextContentPresenter" Grid.ColumnSpan="2" Content="{TemplateBinding PlaceholderText}" Foreground="{ThemeResource SystemControlPageTextBaseMediumBrush}" IsHitTestVisible="False" IsTabStop="False" Margin="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}" Grid.Row="1" HorizontalAlignment="Center"/>
The complete modified style:
<Page.Resources>
<Style x:Key="MyTextBoxStyl" TargetType="TextBox">
<Setter Property="MinWidth" Value="{ThemeResource TextControlThemeMinWidth}"/>
<Setter Property="MinHeight" Value="{ThemeResource TextControlThemeMinHeight}"/>
<Setter Property="Foreground" Value="{ThemeResource SystemControlForegroundBaseHighBrush}"/>
<Setter Property="Background" Value="{ThemeResource SystemControlBackgroundAltHighBrush}"/>
<Setter Property="BorderBrush" Value="{ThemeResource SystemControlForegroundChromeDisabledLowBrush}"/>
<Setter Property="SelectionHighlightColor" Value="{ThemeResource SystemControlHighlightAccentBrush}"/>
<Setter Property="BorderThickness" Value="{ThemeResource TextControlBorderThemeThickness}"/>
<Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}"/>
<Setter Property="FontSize" Value="{ThemeResource ControlContentThemeFontSize}"/>
<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="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" BorderBrush="{ThemeResource TextBoxButtonBorderThemeBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{ThemeResource TextBoxButtonBackgroundThemeBrush}">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="GlyphElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAccentBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="ButtonLayoutGrid">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAccentBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="GlyphElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltChromeWhiteBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<DoubleAnimation Duration="0" To="0" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="ButtonLayoutGrid"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<TextBlock x:Name="GlyphElement" AutomationProperties.AccessibilityView="Raw" Foreground="{ThemeResource SystemControlForegroundChromeBlackMediumBrush}" FontStyle="Normal" FontSize="12" FontFamily="{ThemeResource SymbolThemeFontFamily}" HorizontalAlignment="Center" Text="" VerticalAlignment="Center"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Grid.Resources>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="HeaderContentPresenter">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledBaseLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="BackgroundElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledTransparentBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="BorderElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlBackgroundBaseLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="BorderElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledBaseLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledChromeDisabledLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="PlaceholderTextContentPresenter">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledChromeDisabledLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Normal"/>
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="BorderElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightChromeAltLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Opacity" Storyboard.TargetName="BackgroundElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBackgroundHoverOpacity}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Focused">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="PlaceholderTextContentPresenter">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlPageTextChromeBlackMediumLowBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="BackgroundElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlBackgroundChromeWhiteBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Opacity" Storyboard.TargetName="BackgroundElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBackgroundFocusedOpacity}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="BorderElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAccentBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlForegroundChromeBlackHighBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="RequestedTheme" Storyboard.TargetName="ContentElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="Light"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="ButtonStates">
<VisualState x:Name="ButtonVisible">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="DeleteButton">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="ButtonCollapsed"/>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border x:Name="BackgroundElement" Background="{TemplateBinding Background}" Grid.ColumnSpan="2" Margin="{TemplateBinding BorderThickness}" Opacity="{ThemeResource TextControlBackgroundRestOpacity}" Grid.Row="1" Grid.RowSpan="1"/>
<Border x:Name="BorderElement" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Grid.ColumnSpan="2" Grid.Row="1" Grid.RowSpan="1"/>
<ContentPresenter x:Name="HeaderContentPresenter" Grid.ColumnSpan="2" ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" Foreground="{ThemeResource SystemControlForegroundBaseHighBrush}" FontWeight="Normal" Margin="0,0,0,8" Grid.Row="0" Visibility="Collapsed" x:DeferLoadStrategy="Lazy"/>
<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}" Padding="{TemplateBinding Padding}" Grid.Row="1" VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}" VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}" ZoomMode="Disabled"/>
<!--Here below we modify the horizontal alignment-->
<ContentControl x:Name="PlaceholderTextContentPresenter" Grid.ColumnSpan="2" Content="{TemplateBinding PlaceholderText}" Foreground="{ThemeResource SystemControlPageTextBaseMediumBrush}" IsHitTestVisible="False" IsTabStop="False" Margin="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}" Grid.Row="1" HorizontalAlignment="Center"/>
<Button x:Name="DeleteButton" BorderThickness="{TemplateBinding BorderThickness}" Grid.Column="1" FontSize="{TemplateBinding FontSize}" IsTabStop="False" Margin="{ThemeResource HelperButtonThemePadding}" MinWidth="34" Grid.Row="1" Style="{StaticResource DeleteButtonStyle}" Visibility="Collapsed" VerticalAlignment="Stretch"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Page.Resources>
<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
<TextBox Name="TxtCompanyId" PlaceholderText="FirmaId" Height="40" Margin="5" TextAlignment="Center" Style="{StaticResource MyTextBoxStyl}"/>
</Grid>

How to apply global transform rules in app.xaml in windows phone 8

I want to apply this transform rule to all checkboxs:
<CheckBox.RenderTransform>
<ScaleTransform ScaleX="0.7" ScaleY="0.7" />
</CheckBox.RenderTransform>
Is there some way that I can put it into app.xaml so that I don't need to write this rule to every checkbox?

Sure, all you have to do is override default checkbox style. Add this code to your App.xaml file in <Application.Resources> section:
<!--Application Resources-->
<Application.Resources>
<Style x:Key="PhoneButtonBase" TargetType="ButtonBase">
<Setter Property="Background" Value="Transparent"/>
<Setter Property="BorderBrush" Value="{StaticResource PhoneForegroundBrush}"/>
<Setter Property="Foreground" Value="{StaticResource PhoneForegroundBrush}"/>
<Setter Property="BorderThickness" Value="{StaticResource PhoneBorderThickness}"/>
<Setter Property="FontFamily" Value="{StaticResource PhoneFontFamilySemiBold}"/>
<Setter Property="FontSize" Value="{StaticResource PhoneFontSizeMediumLarge}"/>
<Setter Property="Padding" Value="10,3,10,5"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ButtonBase">
<Grid Background="Transparent">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="MouseOver"/>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentContainer">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneBackgroundBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="ButtonBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneForegroundBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="ButtonBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneForegroundBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentContainer">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneDisabledBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="ButtonBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneDisabledBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="ButtonBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="Transparent"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border x:Name="ButtonBackground" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" CornerRadius="0" Margin="{StaticResource PhoneTouchTargetOverhang}">
<ContentControl x:Name="ContentContainer" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" Foreground="{TemplateBinding Foreground}" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" Padding="{TemplateBinding Padding}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Border>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="PhoneRadioButtonCheckBoxBase" BasedOn="{StaticResource PhoneButtonBase}" TargetType="ToggleButton">
<Setter Property="Background" Value="{StaticResource PhoneRadioCheckBoxBrush}"/>
<Setter Property="BorderBrush" Value="{StaticResource PhoneRadioCheckBoxBrush}"/>
<Setter Property="FontSize" Value="{StaticResource PhoneFontSizeMedium}"/>
<Setter Property="FontFamily" Value="{StaticResource PhoneFontFamilyNormal}"/>
<Setter Property="HorizontalContentAlignment" Value="Left"/>
<Setter Property="VerticalContentAlignment" Value="Center"/>
<Setter Property="Padding" Value="0"/>
</Style>
<Style BasedOn="{StaticResource PhoneRadioButtonCheckBoxBase}" TargetType="CheckBox">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="CheckBox">
<Grid Background="Transparent">
<Grid.RenderTransform>
<ScaleTransform ScaleX="0.7" ScaleY="0.7" />
</Grid.RenderTransform>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="MouseOver"/>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="CheckBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneRadioCheckBoxPressedBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="CheckBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneRadioCheckBoxPressedBorderBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Fill" Storyboard.TargetName="CheckMark">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneRadioCheckBoxCheckBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Fill" Storyboard.TargetName="IndeterminateMark">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneRadioCheckBoxCheckBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="CheckBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneRadioCheckBoxDisabledBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="CheckBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneDisabledBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Fill" Storyboard.TargetName="CheckMark">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneRadioCheckBoxCheckDisabledBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Fill" Storyboard.TargetName="IndeterminateMark">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneRadioCheckBoxCheckDisabledBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentContainer">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneDisabledBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="CheckStates">
<VisualState x:Name="Checked">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="CheckMark">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Unchecked"/>
<VisualState x:Name="Indeterminate">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="IndeterminateMark">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Grid Margin="{StaticResource PhoneTouchTargetLargeOverhang}">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="32"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Border x:Name="CheckBackground" BorderBrush="{TemplateBinding Background}" BorderThickness="{StaticResource PhoneBorderThickness}" Background="{TemplateBinding Background}" HorizontalAlignment="Left" Height="32" IsHitTestVisible="False" VerticalAlignment="Center" Width="32"/>
<Rectangle x:Name="IndeterminateMark" Fill="{StaticResource PhoneRadioCheckBoxCheckBrush}" HorizontalAlignment="Center" Height="16" IsHitTestVisible="False" Grid.Row="0" Visibility="Collapsed" VerticalAlignment="Center" Width="16"/>
<Path x:Name="CheckMark" Data="M0,119 L31,92 L119,185 L267,0 L300,24 L122,250 z" Fill="{StaticResource PhoneRadioCheckBoxCheckBrush}" HorizontalAlignment="Center" Height="18" IsHitTestVisible="False" Stretch="Fill" StrokeThickness="2" StrokeLineJoin="Round" Visibility="Collapsed" VerticalAlignment="Center" Width="24"/>
<ContentControl x:Name="ContentContainer" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" Grid.Column="1" Foreground="{TemplateBinding Foreground}" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="12,0,0,0" Padding="{TemplateBinding Padding}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Grid>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Application.Resources>
I have added scale transformation in checkbox template:
<Grid.RenderTransform>
<ScaleTransform ScaleX="0.7" ScaleY="0.7" />
</Grid.RenderTransform>
I know, it's a lot of XAML code. But you can easily change any aspect of the checkbox control here.

VerticalContentAlignement into TextBox

I have a problem with a style I have created, I'm trying to center the text vertically but it doesn't works. Indeed, if I set the VerticalAlignment property of the ScollViewer to Center the Text of the TextBox, the text is not visible anymore from my TextBox.
My style is define as following :
<Style TargetType="TextBox">
<Setter Property="MinWidth" Value="{ThemeResource TextControlThemeMinWidth}"/>
<Setter Property="MinHeight" Value="{ThemeResource TextControlThemeMinHeight}"/>
<Setter Property="Foreground" Value="{ThemeResource TextBoxForegroundThemeBrush}"/>
<Setter Property="Background" Value="{ThemeResource TextBoxBackgroundThemeBrush}"/>
<Setter Property="BorderBrush" Value="Black"/>
<Setter Property="SelectionHighlightColor" Value="{ThemeResource TextSelectionHighlightColorThemeBrush}"/>
<Setter Property="BorderThickness" Value="2"/>
<Setter Property="VerticalContentAlignment" Value="Center" />
<Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}"/>
<Setter Property="FontSize" Value="20"/>
<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="Template">
<Setter.Value>
<ControlTemplate TargetType="TextBox">
<Grid>
<Grid.Resources>
<Style x:Name="DeleteButtonStyle" TargetType="Button">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Button">
<Grid>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="BackgroundElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxButtonPointerOverBackgroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="BorderElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxButtonPointerOverBorderThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="GlyphElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxButtonPointerOverForegroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="BackgroundElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxButtonPressedBackgroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="BorderElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxButtonPressedBorderThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="GlyphElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxButtonPressedForegroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<DoubleAnimation Duration="0" To="0" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="BackgroundElement"/>
<DoubleAnimation Duration="0" To="0" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="BorderElement"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border x:Name="BorderElement" CornerRadius="0,10,10,0" BorderBrush="{ThemeResource TextBoxButtonBorderThemeBrush}" BorderThickness="{TemplateBinding BorderThickness}"/>
<Border x:Name="BackgroundElement" CornerRadius="0,10,10,0" Background="{ThemeResource TextBoxButtonBackgroundThemeBrush}" Margin="{TemplateBinding BorderThickness}">
<TextBlock x:Name="GlyphElement" AutomationProperties.AccessibilityView="Raw" Foreground="{ThemeResource TextBoxButtonForegroundThemeBrush}" FontStyle="Normal" FontFamily="{ThemeResource SymbolThemeFontFamily}" HorizontalAlignment="Center" Text="" VerticalAlignment="Center"/>
</Border>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Grid.Resources>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="BackgroundElement">
<DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextBoxDisabledBackgroundThemeBrush}"/>
</ObjectAnimationUsingKeyFrames>
<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>
</Storyboard>
</VisualState>
<VisualState x:Name="Normal">
<Storyboard>
<DoubleAnimation Duration="0" To="{ThemeResource TextControlBackgroundThemeOpacity}" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="BackgroundElement"/>
<DoubleAnimation Duration="0" To="{ThemeResource TextControlBorderThemeOpacity}" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="BorderElement"/>
</Storyboard>
</VisualState>
<VisualState x:Name="PointerOver">
<Storyboard>
<DoubleAnimation Duration="0" To="{ThemeResource TextControlPointerOverBackgroundThemeOpacity}" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="BackgroundElement"/>
<DoubleAnimation Duration="0" To="{ThemeResource TextControlPointerOverBorderThemeOpacity}" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="BorderElement"/>
</Storyboard>
</VisualState>
<VisualState x:Name="Focused"/>
</VisualStateGroup>
<VisualStateGroup x:Name="ButtonStates">
<VisualState x:Name="ButtonVisible">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="DeleteButton">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="ButtonCollapsed"/>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border x:Name="BackgroundElement" Background="{TemplateBinding Background}" Grid.ColumnSpan="2" Margin="{TemplateBinding BorderThickness}" CornerRadius="10" />
<Border Grid.Row="0" Grid.Column="0" x:Name="BorderElement" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Grid.ColumnSpan="2" CornerRadius="10" />
<ScrollViewer Grid.Row="0" Grid.Column="0" x:Name="ContentElement" AutomationProperties.AccessibilityView="Raw" HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}" IsTabStop="False" IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}" IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}" IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}" Margin="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}" VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}" ZoomMode="Disabled" VerticalSnapPointsAlignment="Center" VerticalScrollMode="Disabled" HorizontalScrollMode="Disabled" VerticalAlignment="Center"/>
<ContentControl Grid.Row="0" Grid.Column="0" VerticalAlignment="Center" x:Name="PlaceholderTextContentPresenter" Grid.ColumnSpan="2" Content="{TemplateBinding PlaceholderText}" Foreground="{ThemeResource TextBoxPlaceholderTextThemeBrush}" IsHitTestVisible="False" IsTabStop="False" Margin="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}" />
<Button x:Name="DeleteButton" BorderThickness="{TemplateBinding BorderThickness}" Grid.Column="1" FontSize="{TemplateBinding FontSize}" IsTabStop="False" Grid.Row="1" Style="{StaticResource DeleteButtonStyle}" Visibility="Collapsed" VerticalAlignment="Stretch"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Thanks a lot

Transparent ComboBox in WinRT

So I'm trying to use the default ComboBox to display a list of choices. I want the background of the context menu that pops up when the combo box is selected to be Transparent. I can't find a property or style that does this and I haven't found much in the ways of doing this. Thanks in advance!

You need to extract a copy of the current template of your ComboBox with Visual Studio designer view or Blend and modify the Background property of the Border named "PopupBorder" to say Transparent. By default it's using {StaticResource ComboBoxPopupBackgroundThemeBrush}.
Here's a copy of the updated Style resource.
<Style
x:Key="ComboBoxStyle1"
TargetType="ComboBox">
<Setter
Property="Padding"
Value="8,0" />
<Setter
Property="Foreground"
Value="{StaticResource ComboBoxForegroundThemeBrush}" />
<Setter
Property="Background"
Value="{StaticResource ComboBoxBackgroundThemeBrush}" />
<Setter
Property="BorderBrush"
Value="{StaticResource ComboBoxBorderThemeBrush}" />
<Setter
Property="BorderThickness"
Value="{StaticResource ComboBoxBorderThemeThickness}" />
<Setter
Property="TabNavigation"
Value="Once" />
<Setter
Property="ScrollViewer.HorizontalScrollBarVisibility"
Value="Disabled" />
<Setter
Property="ScrollViewer.VerticalScrollBarVisibility"
Value="Auto" />
<Setter
Property="ScrollViewer.HorizontalScrollMode"
Value="Disabled" />
<Setter
Property="ScrollViewer.VerticalScrollMode"
Value="Auto" />
<Setter
Property="ScrollViewer.IsVerticalRailEnabled"
Value="True" />
<Setter
Property="ScrollViewer.IsDeferredScrollingEnabled"
Value="False" />
<Setter
Property="ScrollViewer.BringIntoViewOnFocusChange"
Value="True" />
<Setter
Property="HorizontalContentAlignment"
Value="Stretch" />
<Setter
Property="FontFamily"
Value="{StaticResource ContentControlThemeFontFamily}" />
<Setter
Property="FontSize"
Value="{StaticResource ControlContentThemeFontSize}" />
<Setter
Property="ItemsPanel">
<Setter.Value>
<ItemsPanelTemplate>
<CarouselPanel />
</ItemsPanelTemplate>
</Setter.Value>
</Setter>
<Setter
Property="Template">
<Setter.Value>
<ControlTemplate
TargetType="ComboBox">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition
Width="*" />
<ColumnDefinition
Width="32" />
</Grid.ColumnDefinitions>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup
x:Name="CommonStates">
<VisualState
x:Name="Normal" />
<VisualState
x:Name="PointerOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Background"
Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxPointerOverBackgroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="BorderBrush"
Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxPointerOverBorderThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Fill"
Storyboard.TargetName="Highlight">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxSelectedPointerOverBackgroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState
x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Background"
Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxPressedBackgroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="BorderBrush"
Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxPressedBorderThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Foreground"
Storyboard.TargetName="ContentPresenter">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxPressedForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<DoubleAnimation
Duration="0"
To="1"
Storyboard.TargetProperty="Opacity"
Storyboard.TargetName="PressedBackground" />
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Foreground"
Storyboard.TargetName="DropDownGlyph">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxArrowPressedForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState
x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Background"
Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxDisabledBackgroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="BorderBrush"
Storyboard.TargetName="Background">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxDisabledBorderThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Foreground"
Storyboard.TargetName="ContentPresenter">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxDisabledForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Foreground"
Storyboard.TargetName="DropDownGlyph">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxArrowDisabledForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup
x:Name="FocusStates">
<VisualState
x:Name="Focused">
<Storyboard>
<DoubleAnimation
Duration="0"
To="1"
Storyboard.TargetProperty="Opacity"
Storyboard.TargetName="HighlightBackground" />
<DoubleAnimation
Duration="0"
To="1"
Storyboard.TargetProperty="Opacity"
Storyboard.TargetName="Highlight" />
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Foreground"
Storyboard.TargetName="ContentPresenter">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxFocusedForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState
x:Name="FocusedPressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Foreground"
Storyboard.TargetName="ContentPresenter">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxPressedForegroundThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetProperty="Fill"
Storyboard.TargetName="Highlight">
<DiscreteObjectKeyFrame
KeyTime="0"
Value="{StaticResource ComboBoxPressedHighlightThemeBrush}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState
x:Name="Unfocused" />
<VisualState
x:Name="PointerFocused" />
<VisualState
x:Name="FocusedDropDown">
<Storyboard>
<ObjectAnimationUsingKeyFrames
Duration="0"
Storyboard.TargetProperty="Visibility"
Storyboard.TargetName="PopupBorder">
<DiscreteObjectKeyFrame
KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup
x:Name="DropDownStates">
<VisualState
x:Name="Opened">
<Storyboard>
<SplitOpenThemeAnimation
ClosedTargetName="ContentPresenter"
ContentTranslationOffset="0"
ContentTargetName="ScrollViewer"
ClosedLength="{Binding TemplateSettings.DropDownClosedHeight, RelativeSource={RelativeSource Mode=TemplatedParent}}"
OffsetFromCenter="{Binding TemplateSettings.DropDownOffset, RelativeSource={RelativeSource Mode=TemplatedParent}}"
OpenedTargetName="PopupBorder"
OpenedLength="{Binding TemplateSettings.DropDownOpenedHeight, RelativeSource={RelativeSource Mode=TemplatedParent}}" />
</Storyboard>
</VisualState>
<VisualState
x:Name="Closed">
<Storyboard>
<SplitCloseThemeAnimation
ClosedTargetName="ContentPresenter"
ContentTranslationOffset="40"
ContentTranslationDirection="{Binding TemplateSettings.SelectedItemDirection, RelativeSource={RelativeSource Mode=TemplatedParent}}"
ContentTargetName="ScrollViewer"
ClosedLength="{Binding TemplateSettings.DropDownClosedHeight, RelativeSource={RelativeSource Mode=TemplatedParent}}"
OffsetFromCenter="{Binding TemplateSettings.DropDownOffset, RelativeSource={RelativeSource Mode=TemplatedParent}}"
OpenedTargetName="PopupBorder"
OpenedLength="{Binding TemplateSettings.DropDownOpenedHeight, RelativeSource={RelativeSource Mode=TemplatedParent}}" />
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border
x:Name="Background"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Background="{TemplateBinding Background}"
Grid.ColumnSpan="2" />
<Rectangle
x:Name="PressedBackground"
Fill="{StaticResource ComboBoxPressedHighlightThemeBrush}"
Margin="{TemplateBinding BorderThickness}"
Opacity="0" />
<Border
x:Name="HighlightBackground"
BorderBrush="{StaticResource ComboBoxFocusedBorderThemeBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Background="{StaticResource ComboBoxFocusedBackgroundThemeBrush}"
Grid.ColumnSpan="2"
Opacity="0" />
<Rectangle
x:Name="Highlight"
Fill="{StaticResource ComboBoxSelectedBackgroundThemeBrush}"
Margin="{TemplateBinding BorderThickness}"
Opacity="0" />
<ContentPresenter
x:Name="ContentPresenter"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
Margin="{TemplateBinding Padding}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}" />
<TextBlock
x:Name="DropDownGlyph"
Grid.Column="1"
Foreground="{StaticResource ComboBoxArrowForegroundThemeBrush}"
FontWeight="Bold"
FontSize="{StaticResource ComboBoxArrowThemeFontSize}"
FontFamily="{StaticResource SymbolThemeFontFamily}"
HorizontalAlignment="Right"
IsHitTestVisible="False"
Margin="0,0,6,4"
Text=""
VerticalAlignment="Center" />
<Popup
x:Name="Popup">
<Border
x:Name="PopupBorder"
BorderBrush="{StaticResource ComboBoxPopupBorderThemeBrush}"
BorderThickness="{StaticResource ComboBoxPopupBorderThemeThickness}"
Background="Transparent"
HorizontalAlignment="Stretch">
<ScrollViewer
x:Name="ScrollViewer"
BringIntoViewOnFocusChange="{TemplateBinding ScrollViewer.BringIntoViewOnFocusChange}"
Foreground="{StaticResource ComboBoxPopupForegroundThemeBrush}"
HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}"
HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}"
IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}"
IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}"
IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}"
VerticalSnapPointsType="OptionalSingle"
VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}"
VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}"
VerticalSnapPointsAlignment="Near"
ZoomMode="Disabled">
<ItemsPresenter />
</ScrollViewer>
</Border>
</Popup>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>

Have you tried the following?
<ComboBox.ItemsPanel>
<ItemsPanelTemplate>
<StackPanel Background="Transparent" />
</ItemsPanelTemplate>
</ComboBox.ItemsPanel>
This works for me (without redefining the ControlTemplate).