css linted: selector performance improvement 

I linted the CSS files with CSSLint and adjusted the selectors to make them faster. I tested the changed style sheet and I think we got some performance improvement. 
I was pretty radical, so I can't say for sure that I didn't broke anything, but everything looks fine to me, I just had to fix something with the plus menu. 

There are still some overqualified selectors that could be more effective. Maybe I'll go through every one and look what could get improved later.