I figure theme can be a factor more so for players new to the game when they're considering an alliance (I mean, alliance theme probably attracted quite a few new players to Sparta when 300 was big, and likely still does, for example).
Then again, theme played no part in me joining the NPO when I first started playing (I joined them because I was on red and because they were the big red team alliance) nor when I joined MK (I joined them because it's where most of the other jrcommers migrated to).