Welcome to the modding site. Due to creative's original firmwares' lack of possibilities for customization and modification, some smart guys from the Epizenter and Rockbox communities have finally figured out how to put custom firmware on a lot of the creative players. This means non-standard fonts and themes can be used.

There has already been made a lot of themes for the Zen Vision:m 30 gb (which can be converted to 60 gb themes easily). Themes for other players could also be made, if someone with a player and some creativity are willing to put some time into it.
Themes for the ZVM (zen vision:m) 30 gb version

To change the firmware, you'll usually use the CreativeWizard, although some of the guides use a hacked version of the original Creative updater.
Guide: How to upload firmware
If you want to MAKE themes using the CreativeWizard by Mcuelenaere, read this.

If you just want to upload a premade theme (some of them are really good, see zen vision:m themes) scroll about halfway to the bottom ot the tutorial linked to above to see how to upload the .bin (firmware) files.

Fonts for the zvm (zen vision:m) 30 gb can easily be changed. The easiest way would be to use the CreativeWizard, or follow this pictured guide Great thanks to mcuelenaere for both figuring out how to upload custom firmware and creating his CreativeWizard!