Oh, by no means do you have to write code. If you wanted to, you could. But you don't have to.
To change the themes, just go to System -> Preferences -> Themes
That utility will allow you to change the themes. There are several themes pre-installed. Most of them look like crap.
For more themes, go to
www.gnome-look.org 
Also, Gnome isn't the only GUI for Linux. There are a bunch of others, such as KDE, XFCE, Blackbox/Fluxbox, etc... So if you want to try those out, you can install them from the Add/Remove Apps utility in the Applications menu, then log out, and go to Sessions, choose the one you want to try, and it will load that one instead of Gnome.
To change the login screen, I think you can do that from the themes manager too, but I can't remember. I know you can do it from the login screen .. there is a menu somewhere in there. I haven't used Gnome in a while so I couldn't tell you off the top of my head.
