Try this:
download this file:
http://www.technobullshit.com/diablo5theme.css

Download it under /home/user/.tear/diablo5theme.css (or some other place if you prefer)
Go to Tear > Tools > Options (or push Ctrl+o in tear), there is a line named "user_stylesheet_uri", edit it and change the value to:
file:///home/user/.tear/diablo5theme.css (this has to match where you've put the file, watch out for the 3 / at the beginning)

This CSS theme will put all text areas, input text, ... to white with black text on it. Unless stated otherwise by the visited website. It makes gmail usable.

There are still some things that are not perfect (radio buttons have some black pixels around them), but it makes Tear usable with Diablo5Theme.