<Window x:Class="MainWin.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation"
        Title="MainWindow" Height="350" Width="525">
 
    <telerik:RadDocking x:Name="radDocking" HasDocumentHost="False" AllowUnsafeMode="True">
 
        <telerik:RadSplitContainer InitialPosition="DockedTop" Height="50" Orientation="Vertical">
            <telerik:RadPaneGroup>
                <telerik:RadPane PaneHeaderVisibility="Collapsed">
                    <Grid>
                        <Grid.RowDefinitions>
                            <RowDefinition></RowDefinition>
                            <RowDefinition></RowDefinition>
                        </Grid.RowDefinitions>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition></ColumnDefinition>
                            <ColumnDefinition></ColumnDefinition>
                        </Grid.ColumnDefinitions>
                        <StackPanel Grid.Row="0" Grid.Column="0" Grid.ColumnSpan="2" x:Name="Part1">
                            <telerik:RadButton Content="Office_Blue" Width="100" Height="30" Click="RadButton_Click"/>
                            <telerik:RadButton Content="Expression_Dark" Width="100" Height="30" Click="RadButton_Click"/>
                        </StackPanel>
                        <StackPanel HorizontalAlignment="Stretch" VerticalAlignment="Stretch"  Grid.Row="1" Grid.Column="0"   x:Name="Part2" >
                            <Label Content="Test1" HorizontalContentAlignment="Center" VerticalContentAlignment="Center"/>
                        </StackPanel>
                        <StackPanel Grid.Row="1" Grid.Column="1">
                            <telerik:Label Content="Label 2"  HorizontalAlignment="Center" x:Name="Part3"/>
                        </StackPanel>
                    </Grid>
                </telerik:RadPane>
            </telerik:RadPaneGroup>
        </telerik:RadSplitContainer>
    </telerik:RadDocking>
</Window>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using Telerik.Windows.Controls;
 
namespace MainWin
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            StyleManager.ApplicationTheme = new Expression_DarkTheme();
            //StyleManager.ApplicationTheme = new Office_BlueTheme();
            InitializeComponent();
        }
 
        private void RadButton_Click(object sender, RoutedEventArgs e)
        {
            RadButton btn = sender as RadButton;
            if (btn.Content.ToString() == "Office_Blue")
            {
                SetGlobalTheme(new Office_BlueTheme());
            }
            else
            {
                SetGlobalTheme(new Expression_DarkTheme());
            }
        }
 
 
        public static void SetGlobalTheme(Theme theme)
        {
            StyleManager.ApplicationTheme = theme;
 
            foreach (Window w in Application.Current.Windows)
            {
                SetThemeToVisualObject(w);
            }
        }
   
        public static void SetThemeToVisualObject(DependencyObject myVisual)
        {
 
            foreach (object o in LogicalTreeHelper.GetChildren(myVisual))
            {
                if (o is FrameworkElement)
                {
                    if (o.GetType().AssemblyQualifiedName.StartsWith("Telerik.Windows.Controls"))
                        StyleManager.SetTheme((FrameworkElement)o, StyleManager.ApplicationTheme);
 
                    SetThemeToVisualObject((FrameworkElement)o);
                }
            }
        }
    }
}

StyleManager.ApplicationTheme = new Expression_DarkTheme();

StyleManager.ApplicationTheme = new Office_BlueTheme();