The default colorization included in the Windows 7.theme file is designed for use with Aero Glass by Big Muscle. You must set the caption text color to any non-0 value in the AeroGlassGUI.exe (e.g. 0, 0, 1) for the text to become black.

If you can't/don't want to use Aero Glass by Big Muscle, open the Windows 7.theme file in a text editor, and replace the ColorizationColor value with 0XC431A2F7 (it's on line 55 and under [VisualStyles])