I don't know if it's the right thread for my question...

I'm currently customizing a theme that I downloaded. I changed a lot of PNGs and it works perfectly. But now, background and text colors don't match very well with my modifications. How can I change the frame background color, popup menu background color and title text color, for example? I've noticed a colors.config file and a CSS file. I've edited them both but nothing happens. (By the way, what is the subtle difference between those two files?)

I'm trying to modify the Pleego theme from the Black Plastic app. Since there is a small applet to change settings bundled with this app, maybe editing conf file isn't enough?