[Review] Rewrite style.css to mobile-first design

I made quite a few changes to style.css to lay the groundwork for the eventual mobile/handheld theme(s) that I'll be building later on. I also organized/categorized style.css to hopefully get others to not be so horrified when wanting to make changes.

Some changes (in commit <a href="https://github.com/wzsk/vichan/commit/0de981530abd3fc85528ce65a1b32156332c9aa1">0de9815</a>) might not be very welcome, and I would be happy to hear any opinions and make improvements.

Also, I think it would be best if we discussed the mobile design with users who have great opinions on implementation (ie improvements in contrast with other competing chans).