Quote - (JSinger @ Sep 13 2009, 11:39)

A little more information about your setup would help you get better quality advice. For instance, are you referring to Firefox themes or linux themes? Do you have the same issue with any other apps that displays content with objects containing numerous 'color-spaces' (Such as a word processor document with a variety of embedded charts and images)? What video hardware and its settings for your desktop? If you're running in a high resolution with a 16 bpp, your desktop may be starved for colors since that would only allow for 4096 colors on a 1,920,000 pixel surface. Even 24 bpp only offers 524,288 when using an alpha channel (which is fairly certain).
Or it could be a backwards compatibility issue if you're trying to theme Firefox 3.0.x with Firefox 3.5.x compatible themes.
In any case, try to ensure your display is setup for 32 bpp (16,772,216 colors + 8 bit alpha) and the theme you're installing is designed for the Firefox version you're running (by either moving to Firefox 3.5.x or finding a pre-3.5 dark theme). Even if you have to reduce the resolution of your desktop to get 32 bpp color, you'll at least be able to isolate the issue to hardware/settings vs a Firefox issue.
HTH
~js
Sorry I wasn't specific enough. Here's some extra information that might help: For starters, I'm referring to dark GTK themes. If I switch from, say, Clearlooks to Darklooks, and then open Firefox, all my text boxes and such match the colors of the theme or are completely dark. My panels and windows and such are fine. My resolution is 1400x1050, and, as far as I know, I'm using the highest color setting possible.
I guess what I'm trying to say is: Firefox is trying to adapt to the colors of my GTK theme, but only does so for text boxes, which is really annoying. I'm trying to get websites (I'm assuming that's what controls the colors) to use the page default instead. But no matter which option I select, I'm stuck with the theme's defaults instead.