This may be one of the most disputed game design topics on the internet. I have heard people claim theme is irrelevant, and I have heard people claim theme is the only thing that matters. There are so many conflicting definitions it seems impossible to even clarify what anyone is really talking about. It is a polarizing and divisive topic. Is there any way to make sense of the chaos here?

The Importance of Theme

I used to be one of the people who said the theme didn't matter at all, that a game is entirely its mechanic and the "theme" was merely a cosmetic layer over this fundamental core (this is when I was playing a lot of Go). But over time and after playing many different kinds of games, this view shifted until now I see them both as an inseparable part of evoking an experience.

The experience is why people play games. But what is an "experience"?

Theme and Mechanic

This is where the theme and mechanic meet: they are both in the service of the experience. Having a mechanic alone can be a great game, whereas having a theme alone is not really a game. It is art, it is a story, but not a game. But if the mechanic is in support of a solid theme, if the dynamics are aligned with a narrative and a visual style that really come to life, then you can create an experience you cannot get to with mechanics alone, or theme alone! This is where the theme and mechanic fuse into something greater than the sum of its parts, and the experience emerges from the bits and becomes a thing of the mind. This fusion and this quality are why I now consider the theme to be an essential part of the game experience. There is so much potential to evoke an amazing experience from a game (as opposed to a story or a movie or a painting) because in a game the player is an active participant in how things unfold. In a game the player has agency, and their decisions affect the outcome and become part of the fabric of the story that is told.

The best game I can imagine is not one where afterwards the players simply said "You won by X points" but one where everyone says "I learned something and/or I will remember that forever." Or even "That changed my life". Now THAT would be a great game!