hm. You probably best off reading the mod/xml files from these mods..they describe whats being changed..but I agred it can be a daunting task.
Are you using the latest version of Mercury?
And, yes..I noticed that many places now do use the themes..
I did a quick google on them...:
Apollo - 57 hits
Helios - 34.200 hits
Mercury - 25 hits
Juno - 463 hits
TechHead - 2370 hits
Themis - 2830 hits
Simplicity - 266 hits
TinyPortal - 43 hits
Seem Helios is quite popular...