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.
Merge pull request #52 from KoffeinFlummi/background-colour

Make body have colour scheme's background colour