You can choose to install GNOME when you install Mageia, and then installing additional themes should be very easy. I am not a gnome user myself, but I imagine that you can do it through the GNOME settings (somewhere) or through rpmdrake - the Mageia GUI frontend for installing and removing software packages.
rpmdrake (which can be found in the GNOME menu as Install and Remove Software or in the MCC (Mageia Control Center)) can be used to install just about any application or software package you need, while automatically resolving all dependencies with a few clicks of your mouse. This includes of course VLC and Rhythmbox.
Mageia is very stable, the only times it's crashed on me was when I did it on purpose. However, running heavy graphics on old or insufficient hardware will crash the X graphical server. Mageia recovers gracefully from those crashes. But they are not the fault of Mageia, rather the over-taxation of your system. So unless you're trying to play heavy graphics games in WINE on a 5 year old laptop (like I was) you probably won't have a problem with stability.

As for fonts, again, the MCC has a nice GUI frontend for installing fonts. I personally like the Deja-Vu font set that comes with the distro, but to each his or her own.
I hope you enjoy Mageia, and welcome to the club!