_s: Document background color fallback.

A default background color is needed for any time there is no custom background color.
This happens to use the browser's default `#fff`, and is meant more as a reminder for theme authors.

See #515.
Minor spacing fix for main navigation

Just an itsy-bitsy space removal, no biggie, but needed :)
Merge pull request #517 from emiluzelac/patch-29

Minor spacing fix for main navigation props @emiluzelac
Revert the change to spacing in #517

Adds back in space for now. Props @lancewillett
Merge remote-tracking branch 'upstream/master'

* upstream/master:
  Revert the change to spacing in #517
  Minor spacing fix for main navigation
  _s: Document background color fallback.