Update qunit.css

From a quick look I had I couldn't find any coding style guide for the CSS but maybe I missed it.

I tried to be consistent with the current style, but if people prefer `normal` or color names let me know and I will rebase the PR.

For the vendor prefixed properties we could add https://github.com/nDmitry/grunt-autoprefixer.

BTW, how about generating the minified versions for the files too in dist?