Allow themes to override global menu styles

This will allow theme artists to style the weather menu independently of other menus (as requested by @zagortenay333 in https://github.com/mockturtl/cinnamon-weather/issues/65#issuecomment-40321335 and https://github.com/collinss/cinnamon-places-center/issues/1#issuecomment-41427797)
Merge pull request #66 from collinss/patch-1

Allow themes to override global menu styles