ControlGalleryView.xaml

<ResourceDictionary 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:pctrls="clr-namespace:Playnite.FullscreenApp.Controls;assembly=Playnite.FullscreenApp" 
                    mc:Ignorable="d">

    <Style x:Key="ControlGalleryContentStyle" TargetType="{x:Type ContentControl}">        
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type ContentControl}">
                    <DockPanel Background="{DynamicResource MainBackgourndBrush}"
                               d:DesignWidth="800" d:DesignHeight="700">
                        <StackPanel>
                            <pctrls:ButtonEx Content="Test" Margin="10" />
                            <pctrls:CheckBoxEx Content="Test" Margin="10" />
                            <pctrls:ComboBoxEx Margin="10" />
                            <pctrls:SliderEx Minimum="0" MaxHeight="10" Value="5" Margin="10" />
                            <pctrls:ToggleButtonEx Content="Test" Margin="10" />
                        </StackPanel>
                    </DockPanel>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>