You can definitely use an external library and pull in ResourceDictionaries, images, etc from it. There are a lot of different options for path syntax in XAML but probably the most stable for this type of situation is the form /<assemblyName>;component/<folder>/<fileName>
If you have a project named MyCompany.SharedResources and you want to use a TextStyles.xaml dictionary in the Styles folder merging that into another project would look like:
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="/MyCompany.SharedResources;component/Styles/TextStyles.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>