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C# wpf get rid of margin on top

I am making a mainWindow, the code is as below.
<Window x:Class="ConfigUI.Views.MainUIView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:materialDesign="http://materialdesigninxaml.net/winfx/xaml/themes"
xmlns:local="clr-namespace:ConfigUI.Views"
mc:Ignorable="d"
Title="MainUIView" Height="450" Width="800"
FontFamily="Segoe UI" FontSize="14"
WindowStartupLocation="CenterScreen" WindowStyle="None"
>
<DockPanel>
<DockPanel DockPanel.Dock="Top" Margin="0">
<Image Source="..\Assets\logo.png"
Width="50" DockPanel.Dock="Left"
/>
<StackPanel DockPanel.Dock="Right">
<Button FontFamily="Segoe MDL2 Assets" FontSize="24"
Content="" Width="50" Height="50" />
</StackPanel>
<TextBlock Text="My Cloud" FontSize="24" FontWeight="Bold"
HorizontalAlignment="Center" VerticalAlignment="Center"
/>
</DockPanel>
<Grid DockPanel.Dock="Bottom"></Grid>
</DockPanel>
</Window>
But when I run it, it always shows a little gap on the top that I can't get rid of it.
My question is how to remove the gap on the top of the window? Thanks.

(Solution 1) This way you will loose drop shadow.
You just set AllowsTransparency="True" in your Window Code. This will remove visible border from window.
<Window x:Class="ConfigUI.Views.MainUIView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:materialDesign="http://materialdesigninxaml.net/winfx/xaml/themes"
xmlns:local="clr-namespace:ConfigUI.Views"
mc:Ignorable="d"
Title="MainUIView" Height="450" Width="800"
FontFamily="Segoe UI" FontSize="14"
WindowStartupLocation="CenterScreen" WindowStyle="None"
AllowsTransparency="True">
</Window>
(Solution 2) Drop shadow will intact.
<WindowChrome.WindowChrome>
<WindowChrome CaptionHeight="0" ResizeBorderThickness="5" />
</WindowChrome.WindowChrome>
Full Codes
<Window x:Class="SOWPF.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:SOWPF"
mc:Ignorable="d" WindowStyle="None"
Title="MainWindow" Height="450" Width="800">
<WindowChrome.WindowChrome>
<WindowChrome CaptionHeight="0" ResizeBorderThickness="5" />
</WindowChrome.WindowChrome>
</Window>
Tip*
Add ResizeMode="CanResize"

Try this:
<DockPanel VerticalAlignment="Top">
or this
<DockPanel Margin="0,-4,0,0">

WPF DockPanel not showing bottom component

I'm new to WPF and I'm trying to create a Window with two text boxes, the RichTextBox at the top, which uses most of the available space, and a TextBox at the bottom. My issue is the TextBox at the bottom is not showing. I have it at the bottom of the DockPanel. What am I missing?
Where is the XAML:
<Window x:Class="WpfApp1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WpfApp1"
mc:Ignorable="d"
Title="MainWindow" Height="450" Width="750" MinHeight="400" MinWidth="600" Background="Black">
<DockPanel
Margin="5"
Height="Auto"
Width="Auto">
<RichTextBox
Name="richTB"
IsEnabled="True"
VerticalScrollBarVisibility="Visible"
IsReadOnly="True">
<FlowDocument
Name="flowDoc"
PagePadding="0">
<Paragraph>
<Run Text="Test" Foreground="Red"></Run>
<Run Text="Foo" Foreground="Blue"></Run>
</Paragraph>
</FlowDocument>
</RichTextBox>
<TextBox
Name="textBox"
KeyDown="EnterPressed"
Background="Plum"
DockPanel.Dock="Bottom">
</TextBox>
</DockPanel>
</Window>

You may have to reorder the children of the DockPanel.
The last child fill uses the Last child in the list of children, it isn't based on where the children are laid out within the DockPanel.
<Window x:Class="WpfApp1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WpfApp1"
mc:Ignorable="d"
Title="MainWindow" Height="450" Width="750" MinHeight="400" MinWidth="600" Background="Black">
<DockPanel
Margin="5"
Height="Auto"
Width="Auto">
<TextBox
Name="textBox"
KeyDown="EnterPressed"
Background="Plum"
DockPanel.Dock="Bottom">
</TextBox>
<RichTextBox
Name="richTB"
IsEnabled="True"
VerticalScrollBarVisibility="Visible"
IsReadOnly="True">
<FlowDocument
Name="flowDoc"
PagePadding="0">
<Paragraph>
<Run Text="Test" Foreground="Red"></Run>
<Run Text="Foo" Foreground="Blue"></Run>
</Paragraph>
</FlowDocument>
</RichTextBox>
</DockPanel>
</Window>

You textbox has no content. The dockpanel sizes its components to the needed size, but because it has no content it has no visible size.
Place some text in it and it will show.

The Heading2 style in ModernUI

We're working with the ModernUI framework for our WPF apps. I've come across a style called Heading2, but I cannot find it anywhere in our code, either the XAML or the C# code. Is Heading2 defined somewhere in ModernUI?

Yes, you can reference it in your code as below:
<Window x:Class="WpfApp1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WpfApp1"
mc:Ignorable="d"
Title="MainWindow" Height="300" Width="300">
<Window.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/FirstFloor.ModernUI;component/Assets/TextBlock.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Window.Resources>
<Grid>
<StackPanel HorizontalAlignment="Center" VerticalAlignment="Center">
<TextBlock Text="Hello Heading2!" Style="{StaticResource Heading2}"></TextBlock>
<TextBlock Text="No Heading..."></TextBlock>
</StackPanel>
</Grid>

Windows 10 universal app UserControl in stackpanel

I need to place a custom UserControl into a Stackpannel.
I have this UserControl:
<UserControl
x:Class="ScannerApp.Custom_Controls.LocationAndQuantity"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:ScannerApp.Custom_Controls"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
d:DesignHeight="20"
d:DesignWidth="400">
<Grid Background="White">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="100*"/>
<ColumnDefinition Width="80*"/>
<ColumnDefinition Width="100*"/>
</Grid.ColumnDefinitions>
<Border x:Name="border" Background="Red" BorderThickness="1" HorizontalAlignment="Left" Height="20" VerticalAlignment="Top" Width="143">
<TextBlock x:Name="locationTxt" Text="location" HorizontalAlignment="Center"></TextBlock>
</Border>
<TextBlock x:Name="quantityTxt" Text="quantity" Grid.Column="2" HorizontalAlignment="Center" TextWrapping="Wrap" VerticalAlignment="Top"/>
</Grid>
</UserControl>
and a page with stackpanel
<Page
x:Class="ScannerApp.FindPN___STEP2"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:ScannerApp"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">
<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
...
<StackPanel>
<!--here I want to place the userControls-->
</StackPanel>
</Grid>
</Page>
I tried some sollutions like <controls: ...> //this could not be found by intellisense even or
<my:UserControlName Grid.Column="2" Grid.Row="2" ... />
<Window ...
xmlns:my="clr-namespace:AssemblyName"
...
/>
but I don't have the Window here... I tried to place something similar into the Page, but I don't really know what to type in there.

As said in comments, you've to fix your XAML. If you want to use custom controls you've to tell the compiler where the controls come from.
In case you Controls namespace is
ScannerApp.Custom_Controls
You've to write the Page XAML as
<Page
x:Class="ScannerApp.FindPN___STEP2"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:ScannerApp.Custom_Controls" <!--FIXED HERE-->
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">
<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
...
<StackPanel>
<local:NameOfYourControl x:Name="MyNewControl" /> <!--Properties can be added-->
</StackPanel>
</Grid>
</Page>

XAML changes without effect

I'm trying to put a textblock in a grid element, but it doesn't display in the debug mode. What do I do wrong? Maybe it's caused that I manipulate the window directly by my C# code?
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="400" Width="500" ResizeMode="NoResize">
<Window.Background>
<ImageBrush ImageSource="/WpfApplication2;component/Images/Grass0118_22_S.jpg"></ImageBrush>
</Window.Background>
<Grid>
<TextBlock Margin="10,10,0,0" Foreground="White" FontWeight="Bold">Życia:</TextBlock>
<TextBlock Margin="50,10,0,0" Foreground="White" Text="{Binding Text, ElementName=points}"></TextBlock>
</Grid>
</Window>

Try this
<Grid>
<Stackpanel Orientation = "Horizontal">
<TextBlock Margin="10,10,0,0" Foreground="White" FontWeight="Bold">Życia:</TextBlock>
<TextBlock Margin="50,10,0,0" Foreground="White" Text="{Binding Text, ElementName=points}"></TextBlock>
</Stackpanel>
</Grid>

I've already solved my issue.
I had defined a canvas tag in my C# code and it overwrote the XAML changes. I replaced the XAML code to the following
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="400" Width="500" ResizeMode="NoResize">
<Window.Background>
<ImageBrush ImageSource="/WpfApplication2;component/Images/Grass0118_22_S.jpg"></ImageBrush>
</Window.Background>
<Canvas>
<Grid>
<TextBlock Margin="10,10,0,0" Foreground="White" FontWeight="Bold">Życia:</TextBlock>
<TextBlock Margin="50,10,0,0" Foreground="White" Text="{Binding Text, ElementName=points}"></TextBlock>
</Grid>
<Canvas Name="mycanvas"></Canvas>
</Canvas>
</Window>
and removed the line creating the canvas element from my C# code and everything works now.
Thanks!