ResourceDictionary is the way to go, you can either copy an xaml file containing the resource dictionary between projects or compile them into a DLL you'll reference from your projects.
To reference dictionaries in the same project you add something like this to your App.xaml (in this case I keep my resources in the ControlStyles folder).
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="ControlStyles/Colors.xaml"/>
<ResourceDictionary Source="ControlStyles/Window.xaml"/>
<ResourceDictionary Source="ControlStyles/Button.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
If you compile them into a different dll you can use this syntax (if the styles dll is called StyleAssembly, the word "component" is actually part of the syntax and not a directory name):
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/StyleAssembly;component/ControlStyles/Colors.xaml"/>
<ResourceDictionary Source="pack://application:,,,/StyleAssembly;component/ControlStyles/Window.xaml"/>
<ResourceDictionary Source="pack://application:,,,/StyleAssembly;component/ControlStyles/Button.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>