You could create a WPF User Control Library in Visual Studio and add ResourceDictionary
items where you define your XAML resources to it.
You then add a reference to this WPF User Control Library from your WPF Application project (Project->Add Reference in Visual Studio) and merge the resource dictionaries that are defined in the library in the App.xaml
of your application:
<Application x:Class="WpfApplication1.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApplication4"
StartupUri="MainWindow.xaml">
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/WpfUserControlLibrary1;component/Dictionary1.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
</Application>
Replace "WpfUserControlLibrary1" with the name of the WPF User Control Library and "Dictionary1" with the name of the ResourceDictionary
that you added to this project.