There's a Henry James quote I'm fond of. What is character but the determination of incident? And what is incident but the illumination of character? "Incident" here is a fancy word for plot, and so what he's saying is that plot should come out of characterization, and that characterization should be revealed as a result of the plot. They're not separate things, but the same thing, hopelessly intertwined, impossible to separate or to consider in isolation.
And one reason I'm fond of it is, if you change "character" and "incident" to "theme" and "mechanisms", we're suddenly talking about game design. Theme is an expression of the mechanisms, and mechanisms an expression of theme. This doesn't mean every game needs to have a well-integrated theme; backgammon gets by just fine without one, and most of the euro-y games are, shall we say, lightly themed. I think when people say they don't care about theme, or that the theme doesn't matter, or that the theme can't matter, they're coming from that sort of euro-y background, and I think that's what causes a lot of fissures when they encounter games in a more simulationist tradition.
Part of the problem might be the word "theme", because a simulation game isn't really a game with a "theme", but a game with a subject – a model – an argument. "Theme" conjures up something that you choose to apply over the game – it could be a thin veneer, as in a euro, or it could be a thick coat of the stuff. This is often the case with "thematic" games – usually in fantasy, horror, or science fiction settings – where a lot of its meaning and value is expressed through the art and the card names and the flavor text.
And, you know, I get it. When someone says, "I'm not sealing off an elder god, I'm just rolling dice and hoping for sixes," I get it. "This thing my character has isn't a charm, it's just a plus-one to my die rolls." That effect might (and should!) make sense, there might be cute little rules that tie the theme to the mechanisms, but in the end, it's only string, easy to untangle or snip in two. There's an art and a craft to it, just like there's an art and a craft to making a popcorn movie. Its joys are deeply felt, but ultimately artificial.
So I definitely understand how folks who have that context will think of games primarily as mechanisms, with theme just something you spackle over it, just as a big budget blockbuster is just a lot of green flat surfaces that have been digitally replaced. But this does a disservice to simulation games – wargames, consims, histogames, whatever you want to call them. Because unlike Ameritrash style games, which are just trying to geek out over the cool bits, simulation games seek to make an argument.
This argument is expressed through its model. Personally, I'm particularly interested in models that foreground observation rather than identification – on pulling the levers and watching the systems at work, rather than putting the players in the shoes of historical actors. But I'm also interested in games that expose and literalize systems, since it's the one thing board games can do better than any other form. It's also one of the more useful and important applications of game design, and one where I see the greatest potential for expanding the capabilities of the art form. So I especially take issue with "theme doesn't matter".
One of the most tiring expressions of this sentiment is from folks who will refuse to accept anything as ever being anything more than math and mechanisms. "You're not persuading public opinion, you're just flipping markers; flipping markers will never represent anything other than flipping markers." With these folks, the conversation is a non-starter; whatever it is that you're modeling, they'll say that nothing is being modeled, that it's only mechanisms. That mechanisms are the only way that a game can make an argument, and since mechanisms on their own have no inherent meaning – flipping a counter is only flipping a counter – it turns out games can't really make arguments at all! Checkmate! Or, if they do, they are largely arguments about mechanisms or player incentives, rather than things that matter.
This is one reason why the word "subject" is probably a better fit than "theme", as it might clearly illustrate the folly of separating the two. Separating a game's mechanisms form its subject is a little like separating a piece of writing from its subject, or a film from its subject. Granted, it is possible to study the art and craft of writing in isolation: it's a stylistics approach, or perhaps a phonesthetic one. And with film, it's possible to appreciate the composition of a frame, the rhythm within it, and the rhythm created by the edit. An approach to games that focuses on mechanisms alone – and more than that, dismisses the subject as irrelevant – is in the same vein: loving the words and the construction of the thing without caring about what those words and that construction expresses.
This is actually a longstanding tradition in film criticism! It is also morally indefensible. Birth of a Nation and Triumph of the Will are two "canon" films – though thankfully they're much less "canon" now then they were when I was made to watch them in high school! – which were held up as technical achievements, as aesthetic experiences, as pure composition and editing. Birth especially gets special pleading for its "innovations". Oh, yes, it's awfully problematic, quoth the film studies teacher, but he invented cross-cutting and close-ups, and so it is important and we have to keep showing it.
But none of that is actually true. All of those innovations were made by other, more obscure directors, whose films aren't deeply racist and who didn't cause a resurgence in hate groups and hate crimes. The only way showing a film like Birth is "justified" is by completely separating the subject from its expression. The theme from its mechanisms. Which is impossible, and causes real harm. You see, this cuts both ways: if the subject doesn't matter, so that you can dismiss attempts to seriously engage, then it doesn't matter so that you can lionize things that are monstrous.
It's an attitude that if taken further enables us to dismiss the implicit biases in our culture and its artifacts. Sexism, racism, homophobia, transphobia, classism: they permeate our culture and are expressed both explicitly and implicitly. And folks who point this out are often dismissed as being overly sensitive, or as looking for reasons to be offended: it's only a movie, only a TV show, only a book, only a song, only a game. At its heart, it's an act of compartmentalization, of separating subject and expression.
Now, I'm not trying to draw a clear or straight line from "theme bad, mechanisms good" to the unquestioned assumptions and inherent toxicity of cultural norms, only underlining that the impulse to engage with an art form without engaging with what that art is on about, is, in its most sinister expression, profoundly abhorrent. But this version of it, when we're just talking about whether or not games are saying the things they're saying? Eh, it's often harmless, mostly it just irks me as someone who cares about the form and its potential.
I say "mostly" though because this also cuts both ways. There are games that say atrocious things, but are hand-waved away because "it's just a game". That scantily-clad warrior woman miniature, whose breasts are so large that she qualifies as a Large Creature? What are you upset about, it's just a game, why are you a killjoy? A game based at a historical university where all the people are white men? Welp, we thought that was historically accurate, turned out we're wrong, but they're not really "people" anyway, this is a game and games are just math, theme doesn't matter, and so on, and so forth.
The theme, the subject, its presentation: it matters. Now, it doesn't create meaning on its own. Again: what is character but the determination of incident? And what is incident but the illumination of character? Theme and mechanisms are one and the same.
And it is entirely possible for them to be at odds, either intentionally or, as is often the case, unintentionally. Designers, like all artists, have a responsibility when making their art to think about what that art is saying, and how their art is saying it. This is especially true of course when tackling serious themes and making serious arguments. But it's just as true I think for more "frivolous" games. The theme in a "themeless euro" doesn't necessarily need to be deeply integrated to succeed on its own terms – some games, just like other art forms, can be mere baubles (though gosh, I do wish baubles didn't account for 95% of new games published), but even in those "themeless euros", the theme can matter. For example, look at the ways in which colonialism is made anodyne, implicitly valuing some stories and lives over others.This doesn't mean that one can't engage with these kinds of subjects – depiction is not endorsement, and if art never engages with horrors, it simply allows them to fester – but like all things, it must be approached with care, thoughtfulness, and responsibility.