Set theme for app
in file 'App.xaml' :
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="/Themes/ColourfulDarkTheme.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
Style Windows :
Style="{DynamicResource CustomWindowStyle}"