Windows 7 Aero theme's "greyed out" - no found fix

Double check the Windows 7 "Themes" Service is running.

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. Even if it is started and automatic, try restarting it.

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

Here's the interesting thing, I don't remember having altered the Themes service. My fault for doing this, but I had followed a tip, which I believe caused the issue. If you Right-click on Computer, Properties, left margin Advanced system settings, then Settings button in Performance section. On Visual Effects tab, Adjust for Best performance, you may end up with this issue once you Log Off or Restart. [ I don't recommend that option, or playing too much with de-selecting all but a few of the Custom options.

Later, I realized I wanted my default Windows 7 Toolbar look back! Aero troubleshooting and a Microsoft FixIt attempt did not find the problem. I went back to check ALL default Windows 7 Services and found the Themes service had been set away from Automatic. Hopefully, my learning the hard way may help you avoid going to far in this same way.

I fixed this on my PC based on the answer by @cpufox.

I restarted the Themes service. It was started already, but restarting it fixed immediately. I didn't even have to go back in to the "Personalize" dialog to apply a theme.

Also, my Themes service was set to manual startup. I set this to Auto just in case.

Please try this to see if it fixes your problem:

1. Go to your control panel and click on Performance Information and Tools.
2. When the window pops up, look at the very bottom of the Performance Information and Tools window, in the right-hand side, for a link that says Re-run the assessment and Click on that link
3. Windows will run through a series of test that will cause your display to flicker and flash but that is normal for that assessment test.

Good Luck!