Not sure what ferngolf is talking about, but you can certainly change the theme on a 7100g.
First thing to do is download and install v4.1 for the 7100g. There is a sticky thread at the top of the General BlackBerry Discussion forum that has the download locations for most of the carriers. While Cingular has not issued v.4.1 yet, several other carriers have.
Once you have v4.1, you can download tons of themes. Start with
http://mobile.blackberry.com and download those.
Then search the Ringtones, Themes and Wallpapers forum for the tons of themes that are out there for the 7100.
Good luck.