I totally get the goal here: make CSS more modular and scalable in WordPress. Put all your global WordPress theme styles in a single file, including variations. JSON offers a nicely structured syntax that’s easily consumable by JavaScript, thereby allowing the sweet affordance of loading exactly what we want when we want it.
The problem, to me, is that writing “CSS” in a theme.json file is a complete mental model switcher-oo. Rather than selectors, we have a whole set of objects we have to know about just to select something. We have JSON properties that look and feel like CSS properties, only they have to be camelCased being JavaScript and all. And we’re configuring features in the middle of the styles, meaning we’ve lost a clear separation of concerns.
I’m playing devil’s advocate, of course. There’s a lot of upside to abstracting CSS with JSON for the very niche purpose of theming CMS templates and components. But after a decade of “CSS-in-JS is the Way™” I’m less inclined to buy into it. CSS is the bee’s knees just the way it is and I’m OK relying on it solely, whether it’s in the required style.css file or some other plain ol’ CSS file I generate. But that also means I’m losing out on the WordPress features that require you to write styles in a theme.json file, like style variations that can be toggled directly in the WordPress admin.
Regardless of all that, I’m linking this up because Justin does bang-up work (no surprise, really) explaining and illustrating the ways of CSS-in-WordPress. We have a complete guide that Ganesh rocked a couple of years ago. You might check that to get familiar with some terminology, jump into a nerdy deep dive on how WordPress generates classes from JSON, or just use the reference tables as a cheat sheet.