_s: Document background color fallback.
_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.
vaishaliagola27/_s,andbaz/launchbaz,vaishaliagola27/_s,carl-alberto/business-portfolio,andbaz/launchbaz,vaishaliagola27/_s,madeso-uk/vcsassist,jchck/jchck_,carl-alberto/business-portfolio,jchck/jchck_,madeso-uk/vcsassist