Improve tab styles on small screens

Tabs weren't really styled at all on small screens.

![screen shot 2014-07-07 at 12 47 37](https://cloud.githubusercontent.com/assets/121219/3494769/b04e76d2-05cc-11e4-91c1-42c7fd21d743.png)

This commit rejigs some of the styles so that they are more mobile first, and makes them a little bit tidier on small screens (they're nowhere near perfect, but improved).

![screen shot 2014-07-07 at 12 48 22](https://cloud.githubusercontent.com/assets/121219/3494772/ba9086a8-05cc-11e4-85c8-fcc3cc376fc5.png)

In some cases I haven't styled this page mobile first (eg background and border colours). This is to improve the page for browsers that don't support media queries (IE 8) because it doesn't complicate our code that much.