Theme caches are fun, if maintained and not taken tooooo seriously! We have one Crawfishing that has a fishing theme.
Other theme-caches we have seen and enjoyed were ones that emphasized books, CDs (a CD-only exchange; other trades discouraged but TNLNSL fine), several that feature keychains and coins (US state quarters, foreign coins, etc). Hands-down our favorite though has to be the one that features cartoon art and requests that the cacher either sketch a cartoon in the logbook, or bring a copy of a favorite cartoon! CCC cache VERY creative!
So, yeah, theme-caches are fine -- just recall that this IS a family-safe game/sport/hobby!
I don't think that there is currently a way to flag them as 'theme-caches' except in the title; same as TB Hotels. (We have fun coming up with double-entendre titles, like our Bugs 'N Peaches Welcome You! which is a bug-hotel/exchange cache at a Georgia Welcome Center!