Spent a little time towards the end of last week working with Rich on bug #157962, to get some colour back into gcalctool. We haven’t yet come up with a complete solution, which would involve an option that picked colours from the theme by default (I’m still thinking about that one), but in the meantime you can now copy the cvs-supplied .gcacltoolrc file to your homedir to change this boring old look into something slightly more sexy. (Or, of course, you can invent your own colour scheme to match your GNOME theme du jour.)