Stylesheet updates

Created by: lahwaacz

Continuing #10, I tried to make the stylesheets more generic. There is now a new file arch_definitions.less (for now it contains just the definitions of colors and border styles), which is imported in all other LESS files. Most problematic is the matching of links, which have to be matched carefuly even in the skin's navigation elements, but each skin uses different class names. Not sure what is the best solution...

I also slightly improved the styling of Vector tabs and added comments for future reference.