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<Button
    x:Class="Program.GUI.Controls.FlatButton"
    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:GUIControls="clr-namespace:Program.GUI.Controls"
    mc:Ignorable="d">
    <Button.Resources>
        <ResourceDictionary>
            <Style x:Key="{x:Type GUIControls:FlatButton}" TargetType="{x:Type GUIControls:FlatButton}" BasedOn="{StaticResource ResourceKey={x:Type Button}}">
                <Setter Property="OverridesDefaultStyle" Value="True"/>
                <Setter Property="WindowChrome.IsHitTestVisibleInChrome" Value="True"/>
                <Setter Property="BorderThickness" Value="0"/>
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="{x:Type GUIControls:FlatButton}">
                            <Grid x:Name="LayoutRoot" Background="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Background}">
                                <TextBlock
                                    x:Name="Text"
                                    Text="{TemplateBinding Content}"
                                    Foreground="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Foreground}"
                                    HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                    VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
                            </Grid>
                            <ControlTemplate.Triggers>
                                <Trigger Property="IsMouseOver" Value="True">
                                    <Setter TargetName="LayoutRoot" Property="Background" Value="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=HoverBackground}"/>
                                    <Setter TargetName="Text" Property="Foreground" Value="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=HoverForeground}"/>
                                </Trigger>
                                <Trigger Property="IsMouseCaptured" Value="True">
                                    <Setter TargetName="LayoutRoot" Property="Background" Value="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=ClickBackground}"/>
                                    <Setter TargetName="Text" Property="Foreground" Value="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=ClickForeground}"/>
                                </Trigger>
                            </ControlTemplate.Triggers>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>
        </ResourceDictionary>
    </Button.Resources>
</Button>                    

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using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;

namespace Program.GUI.Controls {
    /// <summary>
    /// Interaction logic for FlatButton.xaml
    /// </summary>
    public partial class FlatButton : Button {
        public static readonly DependencyProperty HoverBackgroundProperty = DependencyProperty.Register(
            "HoverBackground",
            typeof(Brush),
            typeof(FlatButton)
        );

        public static readonly DependencyProperty HoverForegroundProperty = DependencyProperty.Register(
            "HoverForeground",
            typeof(Brush),
            typeof(FlatButton)
        );

        public static readonly DependencyProperty ClickBackgroundProperty = DependencyProperty.Register(
            "ClickBackground",
            typeof(Brush),
            typeof(FlatButton)
        );

        public static readonly DependencyProperty ClickForegroundProperty = DependencyProperty.Register(
            "ClickForeground",
            typeof(Brush),
            typeof(FlatButton)
        );

        public Brush HoverBackground { get; set; }
        public Brush HoverForeground { get; set; }

        public Brush ClickBackground { get; set; }
        public Brush ClickForeground { get; set; }

        static FlatButton() {
            DefaultStyleKeyProperty.OverrideMetadata(typeof(FlatButton), new FrameworkPropertyMetadata(typeof(FlatButton)));
        }

        public FlatButton() {
            this.InitializeComponent();
        }
    }
}                    

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<Style x:Key="DefaultDarkButtonThemeStyle" TargetType="{x:Type GUIControls:FlatButton}" BasedOn="{StaticResource {x:Type GUIControls:FlatButton}}">
    <Setter Property="Background" Value="Transparent"/>
    <Setter Property="Foreground" Value="{DynamicResource ColorTextPrimaryDark}"/>
    <Setter Property="HoverBackground" Value="{DynamicResource ColorPrimaryDarkLight}"/>
    <Setter Property="HoverForeground" Value="{DynamicResource ColorTextPrimaryDarkLight}"/>
    <Setter Property="ClickBackground" Value="{DynamicResource ColorPrimaryDarkLightLight}"/>
    <Setter Property="ClickForeground" Value="{DynamicResource ColorTextPrimaryDarkLightLight}"/>
</Style>                    

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System.Windows.Markup.XamlParseException: ''Provide value on 'System.Windows.Markup.StaticResourceHolder' threw an exception.' Line number '45' and line position '47'.'

Inner Exception
Exception: Cannot find resource named 'Program.GUI.Controls.FlatButton'. Resource names are case sensitive.                    

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Line number '45' and line position '47'