Overriding Styles in Xaml

App.xaml

<Application x:Class="WpfApp1.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:WpfApp1"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <!-- base colors -->
        <SolidColorBrush x:Key="buttonBackground" Color="#ff00ff"></SolidColorBrush>
        
        <!-- base button style -->
        <Style x:Key="baseButtonStyle" TargetType="Button">
            <Setter Property="Margin" Value="10,10,10,10"></Setter>
            <Setter Property="Background" Value="{DynamicResource buttonBackground}"></Setter>
        </Style>

        <!-- automatically style all buttons -->
        <Style TargetType="Button" BasedOn="{StaticResource baseButtonStyle}">            
        </Style>

        <!-- Window style -->
        <Style x:Key="window">
            <Style.Resources>
                <ResourceDictionary>
                    <SolidColorBrush x:Key="buttonBackground">Aqua</SolidColorBrush>
                </ResourceDictionary>
            </Style.Resources>
        </Style>

        <!-- normal primary button style -->
        <Style x:Key="primaryButton" TargetType="Button" BasedOn="{StaticResource baseButtonStyle}">
            <Style.Resources>
                <ResourceDictionary>
                    <SolidColorBrush x:Key="buttonBackground">Blue</SolidColorBrush>
                </ResourceDictionary>
            </Style.Resources>
        </Style>

        <!-- normal secondary button style -->
        <Style x:Key="secondaryButton" TargetType="Button" BasedOn="{StaticResource baseButtonStyle}">
            <Style.Resources>
                <ResourceDictionary>
                    <SolidColorBrush x:Key="buttonBackground">#ffaaaa</SolidColorBrush>
                </ResourceDictionary>
            </Style.Resources>
        </Style>

        <!-- warning dialog with overrides -->
        <Style x:Key="warning" TargetType="StackPanel">
            <Setter Property="Margin" Value="10"></Setter>
            <Setter Property="Background" Value="#ee7600"></Setter>
            
            <Style.Resources>
                <ResourceDictionary>
                    <!-- warning primary button style -->
                    <Style x:Key="primaryButton" TargetType="Button" BasedOn="{StaticResource primaryButton}">
                        <Style.Resources>
                            <SolidColorBrush x:Key="buttonBackground" Color="#660033"></SolidColorBrush>
                        </Style.Resources>
                    </Style>
                    <Style x:Key="secondaryButton" TargetType="Button" BasedOn="{StaticResource secondaryButton}">
                        <Style.Resources>
                            <SolidColorBrush x:Key="buttonBackground" Color="#aa0099"></SolidColorBrush>
                        </Style.Resources>
                        <Setter Property="Margin" Value="10,10,10,10"></Setter>
                    </Style>
                </ResourceDictionary>
            </Style.Resources>
        </Style>
    </Application.Resources>
</Application>

MainWindow.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="350" Width="525"
        Style="{DynamicResource window}"
        >
    <StackPanel>
        <Button Content="Normal Button"/>
        <Button Style="{DynamicResource primaryButton}" Content="Normal Primary Button"/>
        <Button Style="{DynamicResource secondaryButton}" Content="Normal Secondary Button"/>
        <StackPanel
            Style="{DynamicResource warning}"
            >
            <StackPanel.Resources>
                <ResourceDictionary>
                    <SolidColorBrush x:Key="buttonBackground">Green</SolidColorBrush>
                </ResourceDictionary>
            </StackPanel.Resources>
            <TextBlock><Run Text="Warning Message"/></TextBlock>
            <Button Style="{DynamicResource primaryButton}" Content="Warning Primary Button"/>
            <Button Style="{DynamicResource secondaryButton}" Content="Warning Secondary Button"/>
        </StackPanel>
    </StackPanel>
</Window>

MainWindowWithXamlCSS.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"
        xmlns:css="clr-namespace:XamlCSS;assembly=XamlCSS"
        xmlns:cssWpf="clr-namespace:XamlCSS.WPF;assembly=XamlCSS.WPF"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525"
        >
    <Window.Resources>
        <css:StyleSheet x:Key="cssstyle">
            <css:StyleSheet.Content><![CDATA[

/*
To use XamlCSS:
1. Install nuget XamlCSS.WPF 2.0.0-pre1 (ensure enabling preview-packages!)
2. In App-constructor call XamlCss.WPF.Css.Initialize() 
3. EMPTY App.Resources!
*/

/* default styles */
Button {
       Background: #ff00ff;
       &.primary {
            Background: Blue;
        }
        &.secondary {
            Background: #ffaaaa;
        }
}
/* default window style */
Window {
    Button: {
        Background: Aqua;
    }
}
/* styles in a warning dialog */
.warning {
    Margin: 10;
    Background: #ee7600;
    
    Button {
        &.primary {
            Background: Orange;
        }
        &.secondary {
            Background: #aa0099;
        }
    }
}

]]>
            </css:StyleSheet.Content>
        </css:StyleSheet>
    </Window.Resources>
    <StackPanel
        cssWpf:Css.StyleSheet="{DynamicResource cssstyle}">
        <Button Content="Normal Button"/>
        <Button cssWpf:Css.Class="primary" Content="Normal Primary Button"/>
        <Button cssWpf:Css.Class="secondary" Content="Normal Secondary Button"/>
        <StackPanel
            cssWpf:Css.Class="warning"
            >
            <TextBlock><Run Text="Warning Message"/></TextBlock>
            <Button cssWpf:Css.Class="primary" Content="Warning Primary Button"/>
            <Button cssWpf:Css.Class="secondary" Content="Warning Secondary Button"/>
        </StackPanel>
    </StackPanel>
</Window>