I just activated the themes in my Windows XP (usually I work using the classic Win9x look) and I have seen that two panels are pure black. Other panels are ok (color = clBtnFace). What these 2 panels have in common is their parent. Both are placed directly on a TForm. If I move them to a different parent (another panel for example) the have the appropriate color.
Delphi 7, win xp pro
This is another grandiose Delphi bug. I started do delete stuff from my project and recompile the project. After one hour I finally went to the main form. I deleted the XPManifest control and now, magically, the program works.
The new issue (XPMan) is discussed here: TPanel color is black when I drop a XPManifest control on my form
Edit: Removing the XPMan is not enough. You will have to delete also the RES files. For large applications it may take a while to manually reconstruct that file. Maybe it works to big into that file and manually remove the resources added by XPMan.