Extending Default Style of Parent Control in CustomUserControl not working - wpf

Question

I am trying to extend an implicit Style of a control which is defined in any parent control within a custom UserControl. This doesn't seem to work as I would expect.
The scenario would be: Including a ResourceDictionary in MainWindow.xaml which defines a default theme. Than I want to extend a base Style which is defined in that theme within my UserControl.
What I have already tried:
Having a MainWindow.xaml which defines the following DefaultStyle in its Resources:
<Window x:Class="UserControl_Style_Inheritance.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:UserControl_Style_Inheritance"
mc:Ignorable="d"
Title="MainWindow" Height="450" Width="800">
<Window.Resources>
<Style TargetType="{x:Type Button}" BasedOn="{StaticResource {x:Type Button}}">
<Setter Property="Background" Value="Blue"></Setter>
</Style>
</Window.Resources>
<Grid>
<local:MyUserControl></local:MyUserControl>
</Grid>
</Window>
And having a MyUserControl.xaml which should extend the defined DefaultStyle of MainWindow.xaml:
<UserControl x:Class="UserControl_Style_Inheritance.MyUserControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:UserControl_Style_Inheritance"
mc:Ignorable="d"
d:DesignHeight="450" d:DesignWidth="800">
<UserControl.Resources>
<Style TargetType="{x:Type Button}" BasedOn="{StaticResource {x:Type Button}}">
<Setter Property="BorderBrush" Value="Red"></Setter>
</Style>
</UserControl.Resources>
<Grid>
<Button>Hello</Button>
</Grid>
</UserControl>
But MyUserControl is based on the normal Button Style by having a gray background instead of a blue background of the Style defined in MainWindow.xaml.
What am I doing wrong, or where am I thinking into the wrong direction?
Edit: Putting the default Style into the Application.xaml file works, but this is not what I want to do.

Answer 1

Try adding the button style to App.xaml:
<Application.Resources>
<Style x:Key="BtnStyle" TargetType="{x:Type Button}" BasedOn="{StaticResource {x:Type Button}}">
<Setter Property="Background" Value="Blue"></Setter>
</Style>
</Application.Resources>
And use it in your UserControl like this:
<UserControl.Resources>
<Style TargetType="{x:Type Button}" BasedOn="{StaticResource BtnStyle}">
<Setter Property="BorderBrush" Value="Red"></Setter>
</Style>
</UserControl.Resources>
<Grid>
<Button>Hello</Button>
</Grid>