I use the Expression Dark Theme in my WPF application.
Also I have to use Extended WPF Toolkit Controls.
How I can apply this theme to them?

XAML:
<Application x:Uid="Application_1" x:Class="Mega.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
StartupUri="MainWindow.xaml"
SessionEnding="App_SessionEnding" >
<Application.Resources>
<ResourceDictionary x:Uid="ResourceDictionary_1" >
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary x:Uid="ResourceDictionary_3" Source="/Themes/ExpressionDark.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
</Application>
You should create your own styles for each custom control from extended toolkit, because standard themes support controls from .NET Framework like button textBox etc, look into the xaml code of theme and create your style in the same way.