ModernWpfPlayground/App/App.xaml

<Application
    x:Class="ModernWpfPlayground.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:ui="http://schemas.modernwpf.com/2019"
    StartupUri="MainWindow.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ui:ThemeResources />
                <ui:IntellisenseResources Source="/ModernWpf;component/DesignTime/DesignTimeResources.xaml" />
                <ui:XamlControlsResources />
                <ResourceDictionary>
                    <Style BasedOn="{StaticResource {x:Type TextBox}}" TargetType="TextBox">
                        <Style.Triggers>
                            <Trigger Property="IsReadOnly" Value="True">
                                <Setter Property="Background" Value="{DynamicResource SystemControlDisabledChromeMediumLowBrush}" />
                            </Trigger>
                        </Style.Triggers>
                    </Style>
                </ResourceDictionary>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>