Doing this as mostly a fun little exercise. Anyway, here are currently the most popular themes on the forums. Be interested to see how this changes in, for example, a year's time.
1. Godzilla (Dark Blue)
2. Mecha-King Ghidorah (Blue)
3. Gotengo (Purple)
4. Godzilla Fire (Dark Black)
5. 20th Century Boys (Classic)
6. Bloodthirsty (Black)
7. Pokémon (Orange)
Also, feel free to use this thread to state which theme you currently use.
Right now, I'm using the Gotengo one as it feels like a nice upgrade of the older forum's design, which I was fond of anyway.
If it bites... don't mess with it!