Possibly some help here:

Some folks, including me, followed a performance tweak and were stuck with the Windows 7 toolbar being a white (Windows Classic). Altering the theme did not correct this. It turned out the Themes service was not automatic.

If you have this problem, where Desktop (right-click) "Personalize" "Windows Color" showing only as Windows Classic).

Run "services.msc", ensure the "Themes" service is Automatic (and Started). That is the Windows 7 default mode for this Service.

Then you can then select any Windows Aero theme and it will provide the toolbar transparency again