"Mozilla’s Thunderbird is a huge pain when it comes to customizing the appearance. I guess because they are Linux junkies they insist on making it almost impossible to change that default 8pt Times Bold font that they use for everything.
"I did finally manage to get all that changed, but contrary to what you will find on the forums on the site, you must place the userchrome.css in the chrome folder inside your profile folder. Not in the profile folder itself, as I found on most of the forum entries..."