The theme of the game is its soul; the mechanics are its body.
Quotes about theme
A fitting theme is not only important for player immersion, but also helps in teaching the game, as it is much easier to learn a game when the theme and mechanisms match.
I get the feeling that there are a lot of games that lie. I don’t like a game that lies. […] Sometimes you buy a game because you think that you’re going to kill dragons and save princesses. And finally all you end up doing is placing tiles on a board, with small cubes on them. There is something I find quite disturbing in this.
If a game is abstract, the rules should be brief. Rules can only be substantial if they encompass a theme.
The mechanics are the abstract means by which the game works. The metaphor is the game’s beautiful lie, the fiction that gives the game context and a broader meaning.