Implementations of Theme
What is it that makes a game have theme? Is it the bits? The box? The images and graphics? If you answered "yes" to any of these, then the theme is in your head. There's nothing wrong with this. I'm sure many gamers would enjoy the most abstract games (eg Risk) even more if the bits were in the shapes of their favorite themes (eg fantasy armies, spaceships, Jedi, etc.).
So when, in fact, is the theme realized in the game play?
You are designing a fantasy-themed game of conflict with fighters and thieves. Fighters deal 4 damage, and thieves deal 2 damage. Thematic, right? Now let's make it about space combat. Heavy starships deal 4 damage, and light deal 2. How about Jedi? Masters deal 4, and apprentices deal 2.
My point is that taking a theme and "abstracting out the numbers" is not really implementing that theme. You are--at best--creating a system that uses theme as metaphor. The systems in the game map to some fictional or real world theme in a way that helps you learn and remember them. The abstracted core could to mapped onto any number of other themes. This leads to the inevitable "pasted on theme" comments. Again, there's nothing wrong with games of this kind.
So what does it really mean to implement a theme? Is it possible to be taught a game using no metaphor, using only abstracted bits, yet have the game play be still unmistakably linked to something so strongly that most players immediately spot the connection? And if so, what designs do this the best?