Xaml Design View can't find Background Brush

in order to clean up my code, I'm trying to split my app.xaml into seperate resource dictionaries. This works at runtime, but not at design time:

snipped in app.xaml

    <Application.Resources>
      <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="/;component/Theme/Colors.xaml" />
            <ResourceDictionary Source="/;component/Theme/Styles.xaml" />
        </ResourceDictionary.MergedDictionaries>
      </ResourceDictionary>
    </Application.Resources>

Colors.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

  <SolidColorBrush x:Key="backgroundBrush" Color="Gold"/>
</ResourceDictionary>

Styles.xaml

    <ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

      <Style TargetType="StatusBar">
        <Setter Property="Background" Value="{StaticResource backgroundBrush}" />
      </Style>
    </ResourceDictionary>

Snipped of MainWindow.xaml

<Window x:Class="test.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="test" Width="800" Height="600" >
    <StatusBar Name="statusBar" DockPanel.Dock="Bottom">
        <StatusBarItem Content="{Binding statusMessage}" />
    </StatusBar>

DesignView gives the error: Error 8 '{DependencyProperty.UnsetValue}' is not a valid value for property 'Background'. C:\Daten\DotNet\test\test\MainWindow.xaml 123

If I put backgroundBrush directly into app.xaml like so:

    <Application.Resources>
      <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="/;component/Theme/Colors.xaml" />
            <ResourceDictionary Source="/;component/Theme/Styles.xaml" />
            <ResourceDictionary>
                <SolidColorBrush x:Key="backgroundBrush" Color="Gold"/>
            </ResourceDictionary>
        </ResourceDictionary.MergedDictionaries>
      </ResourceDictionary>
    </Application.Resources>

DesignView has no problems.

So is there a way to tell DesignView where to find backgroundBrush, if this brush is placed into a seperate resource dictionary?

Thats the problem with StaticResource isnt it. It needs the resource key resolved explicitly using shared \ merged \ direct resource dictionaries hierarchically up.

There are two options...

merge Colors.xaml dictionary in Styles.xaml

OR

in Styles.xaml refer the bursh using DynamicResource.