App.xaml

<Application
    x:Class="WpfTestApp.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:WpfTestApp"
    xmlns:md="http://materialdesigninxaml.net/winfx/xaml/themes"
    StartupUri="MainWindow.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <!--  material design styles  -->
                <!--<ResourceDictionary Source="pack://application:,,,/MaterialDesignThemes.Wpf;component/Themes/MaterialDesign3.Defaults.xaml" />-->
                <ResourceDictionary Source="pack://application:,,,/MaterialDesignThemes.Wpf;component/Themes/Generic.xaml" />
                <ResourceDictionary Source="pack://application:,,,/MaterialDesignExtensions;component/Themes/Generic.xaml" />
                <ResourceDictionary Source="pack://application:,,,/MaterialDesignExtensions;component/Themes/MaterialDesignLightTheme.xaml" />
                <!--<ResourceDictionary Source="pack://application:,,,/MaterialDesignExtensions;component/Themes/MaterialDesignDarkTheme.xaml" />-->
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>