You might create a common styles file in one of your project. For example, this might be a GenericStyles.xml file containing your styles and resources.
In other projects, you could add this file as link: right click on a project -> Add -> Existing Item... -> select GenericStyles.xml file -> open the drop-down menu on the Add button -> select Add As Link.
So now you have a single file that will be part of all your projects.
In each project's app.xaml, you could add your styles and resources using this markup:
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="GenericStyles.xml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
Now, you would have to change only one file in the "main" project, and the other projects would automatically apply these changes.