You simply download a given theme from Gnome-Look. Then go into system/preferences/appearance. Then choose to install a new theme using the "install" button.
Navigate to the download and click install. Mate will take care of the rest. That's not hard to do. Of course, however, many of those themes are built by enthusiastic amateurs and so some of them can be a bit flaky. You just have to fish around for one you like. Or not.
If, on the other hand you do consider it to be too hard, then maybe MS Windows might be your best choice as an OS as it does tend to do literally everything for you......Just so long as you don't mind the suffocating embrace that comes along with it.