I've unlocked four so far. They appear to unlock based on the number of "units" you produce. The fourth theme seemed to take longer than the previous three, but I've been playing a long time without unlocking anything else. Am I just driving myself crazy? Without Game Center support for achievements or leaderboards, unlocks are kind of my only motivation to play this game, as wonderful as it is.
There are a total of 20 Themes to unlock, with the last one being a sort of gag theme, unlocked at 1 million Units Shipped. The 19th, I believe is unlocked at 450K or 500K, I can't remember exactly what we set it at last. The rest are interspersed between that and 1K, with no set interval between them all. IE, the more you unlock, the longer it takes to unlock the next. I think, however, the biggest gap between two themes is 50K.
Also to note, some of the themes are based on other video game properties. 2nd theme is Assassin's Creed 1, with Metroid being another. There may be more, but I forget.