(I will try a better description, as requested by Tim Hunt)
The forum module sends copy of discussion topics to people who has subscribed the forum. The mail sent is an html page. In this page the head section is empty, so no css link is reported. Subscribers receive text and images, but don't receive neither box nor font information nor colors nor any style information.
I checked the problem subscribing a forum of moodle.org: I received only text without any css information.
Add the style information manually in the discussion topic html doesn't solve the problem: they are automatically deleted by moodle.
The problem is probably due to moodle/mod/forum/lib.php line 1071.
Moodle 1.9 was not affected by this problem. May be that to copy some line of code from Moodle 1.9 and paste them in Moodle 2.1 is enough.