<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();

<Style x:Key="TextBox" TargetType="{x:Type my:BaseNumericBox}">

<Setter Property="SnapsToDevicePixels"

Value="True" />

<Setter Property="OverridesDefaultStyle"

Value="True" />

<Setter Property="KeyboardNavigation.TabNavigation"

Value="none" />

<Setter Property="FocusVisualStyle"

Value="{x:Null}" />

<Setter Property="MinWidth"

Value="120" />

<Setter Property="MinHeight"

Value="20" />

<Setter Property="AllowDrop"

Value="true" />

<Setter Property="Template">

<Setter.Value>

<ControlTemplate TargetType="{x:Type my:BaseNumericBox}">

<Grid>

<Border Name="Border"

CornerRadius="2"

Padding="2"

BorderThickness="1">

<Border.Background>

<SolidColorBrush Color="{DynamicResource TextBoxControlLightColor}" />

</Border.Background>

<Border.BorderBrush>

<SolidColorBrush Color="{DynamicResource TextBoxBorderMediumColor}" />

</Border.BorderBrush>

<VisualStateManager.VisualStateGroups>

<VisualStateGroup x:Name="CommonStates">

<VisualState x:Name="Disabled">

<Storyboard>

<ColorAnimationUsingKeyFrames Storyboard.TargetName="Border"

Storyboard.TargetProperty="(Panel.Background).

<EasingColorKeyFrame KeyTime="0"

Value="{DynamicResource TextBoxDisabledControlLightColor}" />

</ColorAnimationUsingKeyFrames>

</Storyboard>

</VisualState>

<VisualState x:Name="Normal">

<Storyboard>

<ColorAnimationUsingKeyFrames Storyboard.TargetName="Border"

Storyboard.TargetProperty="(Panel.Background).

<EasingColorKeyFrame KeyTime="0"

Value="{DynamicResource TextBoxControlLightColor}" />

</ColorAnimationUsingKeyFrames>

</Storyboard>

</VisualState>

<VisualState x:Name="ReadOnly">

<Storyboard>

<ColorAnimationUsingKeyFrames Storyboard.TargetName="Border"

Storyboard.TargetProperty="(Panel.Background).

<EasingColorKeyFrame KeyTime="0"

Value="{DynamicResource TextBoxDisabledControlDarkColor}" />

</ColorAnimationUsingKeyFrames>

</Storyboard>

</VisualState>

<VisualState x:Name="MouseOver" />

</VisualStateGroup>

</VisualStateManager.VisualStateGroups>

<ScrollViewer Margin="0"

x:Name="PART_ContentHost" />

</Border>

</Grid>

<ControlTemplate.Triggers>

<MultiTrigger>

<MultiTrigger.Conditions>

<Condition Property="IsFocused" Value="False"></Condition>

<Condition Property="Text" Value=""></Condition>

</MultiTrigger.Conditions>

<MultiTrigger.Setters>

</MultiTrigger.Setters>

</MultiTrigger>

<Trigger Property="IsFocused" Value="True">

<!--<Setter Property="BorderThickness" TargetName="BorderBase" Value="2.4,2.4,1,1"></Setter>-->

</Trigger>

<Trigger Property="IsEnabled" Value="False">

<Setter Property="Foreground" Value="DimGray" />

<!--<Setter Property="FontStyle" Value="Italic" />-->

</Trigger>

</ControlTemplate.Triggers>

</ControlTemplate>

</Setter.Value>

</Setter>

</Style>