Also > it is also not possible to theme the settings areas. I don't see a reason why this should not work. If by settings area the author means the settings p

Also > it is also not possible to theme the settings areas. I don't see a reason why this should not work. If by settings area the author means the settings page which in modern Thunderbird is more or less a web page in the content area, it should be stylable with userContent.css instead of userChrome.css. The hard part is to find the right @-moz-document selectors for each individual content page.