UPDATE#2 – I added aero folder permission fix. Please try again and let me know if it works.
After getting numerous reports about theme installation error, I tried to investigate and find possibility to resolve this issue. This patcher is the result from my research. Since I can’t reproduce the bug myself, I can’t confirm if it’ll work out for sure so please let me know if you find it working or not. Screenshot of Windows\Resources\Themes\aero folder would be very helpful.
Download: El Capitan UX Patcher