Installing 3rd party themes on Windows 7 x64

I am wishing to install 3rd party themes on my Windows 7 x64 and am wondering if anyone has any views/comments on this both good and bad.

I understand that I will need to use ‘Universal Theme Patcher’ and that it will modify the following files… uxtheme.dll, themeui.dll, themeservice.dll

Of course I will need to make a backup of these files first so where do I locate them?

Could this cause issues in the future if I do this?