Add "theme=dark" to susidns config page, or "susimail.theme=dark" to
susimail.config file, to enable the dark themes.

FIXME: add horizontal scrollbar detection so only adding the extra height
when it is actually needed.

Also added comments to the JS to point out that the issue is that
offsetHeight doesn't include the very top or bottom margins.

(and a tweak to dark console theme)