Remove overly specific css from main.css in site_template

I noticed the main.css file includes some overly specific CSS selectors. For example, since the whole page is wrapped in a div with a class of `site` there is no need to prefix any of the selectors `.site` (apart from the `.site` selector itself). And in general, there is very rarely a compelling reason to qualify a selector with a tag name i.e. it's better to write `.posts` instead of `ul.posts`. 

More info about CSS efficiency can be found over at CSS Wizardry: http://csswizardry.com/2012/07/quasi-qualified-css-selectors/