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
Remove console.log