Improved Scroll Sync and added preview theme css

Hi I added some changes that might be useful to all users. Tried to improve the sync between the editor and the preview so when working at the bottom of a document you can see what you are typing. Also i added a preview theme selector which would need more work as it just uses a raw css file but think it's useful to have separate style sheets for different preview styles (oh and also useful to be able to select your preview style)

cheers