Change <main> bg to white
Change <main> bg to white

This fixes cg-deck's layout so it's main content's background is
white rather then dark blue.

The cg-docs site requires https://github.com/18F/cg-docs/pull/245
for this to not cause a minor color issue on the site.

18F/cg-style,18F/cg-style,18F/cg-style