Simple: is there any way to keep a theme from doing either of those? I can change them back, and some themes even save them, but I just want to keep either from occuring when I try a theme that includes those changes.

When you installed LS Cerbie, you had the option to prevent themes from changing your system colours. If you failed to use this option, just open up personal.rc and follow the instructions in the 'personal settings: syscolor' section.

If the wallpaper changing is being done by the theme through LSThemeInstaller, just run LSThemeInstaller, click the 'Settings' button, and choose not to apply wallpaper. (Note: after 'OK'ing the settings, you can click 'Cancel' to get out of the theme installer without installing a theme, your changes will still be saved.)

There are LiteStep modules that change the wallpaper too though - you'll have to read their documentation, or just comment them out to disable them.

I didn't know about the syscolor bit in the personal.rc, and I did check it for wallpapers in the installer. I'm currently looking through the docs of the few modules I've found that change the wallpaper.
Thanks!