Windows 7 comes with some helpful tools for troubleshooting Aero themes, one of those troubleshooting tools can help you to identify problems with your current display settings that may prevent you from enabling and running Aero Themes on your PC.
Still, if you find that you are unable to apply the Windows 7 or Windows Vista default theme and that in spite of applying it, you get to see The Windows Classic theme, here are a few things you may try:
Windows 7 Aero Theme does not work
1) Type Services.msc in Windows start menu search box and hit Enter to open Services. Scroll down to Themes service, and ensure that it is Started and set on Automatic.
If not, set it so by right-clicking on it and opening the Properties box.
2) Right click on desktop > Personalize > Windows Color and Appearance > Ensure that transparency is checked > OK.
Reboot and try to apply the Theme.
3) Also check this post on how to Flush the Aero cache.
If it still doesn’t help, I suggest you create a new User Profile and see if it resolved the issue. If it does solve your problem, you may transfer Personal Data and continue to use this profile.