[#889] Bases: make sure .comment has a background color

* add a background color to  .comment so that the text is still readable
  in deeply nested comment threads

* remove the background color from .partial .comment because that's now
  redundant

Fixes #889.
Merge pull request #902 from afuna/bases-comment-bgcolor-bug-889

[#889] Bases: make sure .comment has a background color