Exactly how does it fit in? The Windows 7 default theme is Aero, which looks nothing like CCleaner v5.07.5261. The Windows 8 default theme is a Metro-like Aero with no blur or transparency, which looks nothing like CCleaner v5.07.5261. The Windows 10 default theme is an even more modified version of the Windows 8 Aero theme, which looks nothing like CCleaner v5.07.5261. I'm sorry, are you being delusional?
Why did you change the UI? If you left it the old way, it would have fitted well on Win7, Win8 and Win10, now it doesn't fit on either. That's like the dumbest thing you could ever do. Instead of improving the application, you waste time and resources on a UI update which doesn't even make navigation easier, it just makes the whole application look out of place.