Make body have colour scheme's background colour

Make the body have the main background colour of the appropriate colour scheme. This makes a page with a short bit of code (like a one-line patch for instance) look a lot nicer, especially when using a dark theme, as it reduces the contrast between the page and the code.

**Before:**
![](http://i.imgur.com/XjuxKHw.png)

**After:**
![](http://i.imgur.com/qs9iK99.png)