I've written a crappy extension to load an internal website we use for
calendering in a thunderbird tab (loosely based on the
googlecalendartab extension). This works fine (but i still haven't
figured out how to change the menu text or add a toolbar button, but
I'm sure I'll figure that out eventually).

The issue with this calendering app is that it made some poor colour
choices, which i fixed by using a userContent.css file in Firefox. I
put this same userContent.css file in ~/.mozilla-thunderbird/profile/
xxxxxx.default/chrome , but it has no effect there. Is there something
special I have to do to make userContent.css in Thunderbird
(specifically in the ContentTab things)?