[#834] Background color to comments in some layouts

This makes sure that the comment text is readable once they extend over
to the right, and away from the page background

In some cases, add a little padding around the comments so that they
have some space to breathe when they're over the page background

Removes padding from #primary .inner, but adds it back via other
elements, so that there's not doubled padding around comments that are
still over #primary
Merge pull request #849 from afuna/comment-bgcolor-bug-834

[#834] Background color to comments in some layouts