I have recently upgraded to VS 2012, I had to as I needed to start using the .net 4.5 but that is besides the point. My problem is the following:
I have a Resource
it is possible to use the relative path to your appStyles.xaml folder. hope this helps.
something like below
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="../Resources/Styles.xaml"></ResourceDictionary>
</ResourceDictionary.MergedDictionaries>
<ResourceDictionary>