I would create a WPF Custom Control Library, then create a ResourceDictionary to hold the resources, where in the App.xaml of your start-up project I would link the Resource dictionary.
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/myResourceLibrary;component/myResourceDictionary.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
That way, you can just move the DLL around to whichever solution you want to use it in.