Here are the link's to all of the commonly used Theme's that i can find. I figured i would put them all in one post to make it easier for people to navigate to them. Hope this helps, Enjoy!
[Theme] Revolution Theme ~V3~ [Updated 11/10/10] - xda-developers
[Theme] Dark Gloss Mod UPDATED 11/08 Now available in ROM Manager! - xda-developers
[Theme] Nexdeep With AccBatt Mod - xda-developers
[Theme] Singularity (Nexus S Concept) - xda-developers
[THEME] Blue Minimalism with circle battery mod - xda-developers
[THEME][PORT] MattedBlues V4.5 Beta reboot/batt mod 11/13 - xda-developers
[Theme] Storms Blackish Mod-Theme w/ Power Mod & Accurate Battery[Updated 11/07/2010] - xda-developers
[THEME] Blackmod Enhanced *with 15 sec softkey backlight and power options* - xda-developers
[THEME][PORT] Harmony Theme - xda-developers
[THEME][PORT] ::ANDRODeNa GaLaSSiA:: - xda-developers
[Theme] Remix Incognito/Minimal Matte [Updated: 10/31] - xda-developers
All of the Theme's here are credited to their respected developer's. I would suggest reading about which one you wish to try before trying them, to see if it would answer any questions you might come up with.
[Theme] Revolution Theme ~V3~ [Updated 11/10/10] - xda-developers
[Theme] Dark Gloss Mod UPDATED 11/08 Now available in ROM Manager! - xda-developers
[Theme] Nexdeep With AccBatt Mod - xda-developers
[Theme] Singularity (Nexus S Concept) - xda-developers
[THEME] Blue Minimalism with circle battery mod - xda-developers
[THEME][PORT] MattedBlues V4.5 Beta reboot/batt mod 11/13 - xda-developers
[Theme] Storms Blackish Mod-Theme w/ Power Mod & Accurate Battery[Updated 11/07/2010] - xda-developers
[THEME] Blackmod Enhanced *with 15 sec softkey backlight and power options* - xda-developers
[THEME][PORT] Harmony Theme - xda-developers
[THEME][PORT] ::ANDRODeNa GaLaSSiA:: - xda-developers
[Theme] Remix Incognito/Minimal Matte [Updated: 10/31] - xda-developers
All of the Theme's here are credited to their respected developer's. I would suggest reading about which one you wish to try before trying them, to see if it would answer any questions you might come up with.