> You know that you can create themes in GTK (a Desktop GUI) using CSS nowadays, right?

Does GTK offer CSS layouts? Do I describe the elements of the UI using HTML? As far as I know they just use a subset of CSS - aren't the selectors predefined by the framework? I don't recall, exactly. But GTK provides an entirely different set of primitives, I don't think it's the same problem at all if you're just providing fonts and colours. Again, I'm talking about something rather specific, creating complex, desktop-like user interfaces using HTML and SPA frameworks.

> behavior (how it works) and presentation (how it looks) are still separate concerns.

Yeah, but I wouldn't say that that distinction maps to the distinction between CSS and HTML at all, not in my context. HTML, especially document structure, is also presentational. That's really what I mean when I say it's not a "document".

And any way, no matter which approach (and platform, I'd wager) you use, if you want to completely restyle a complex GUI application like what I'm writing in the way you could restyle HTML documents like the Zen garden, you're looking at some serious engineering effort in keeping the so-called behavioural HTML and Javascript in check and documented. It won't just be about how you handle the CSS, it'll be about how you handle everything.

> Yeah, others have though - both as consumers and as integrators. If you are okay with the idea of re-inventing wheels every time you want to have a different color, plenty of people are not.

I actually feel like I have better control of my colours with Tailwind than I ever did. Currently I'm using a limited selection specified in the Tailwind theme. They have names like $adjective-$colour, so it'd be weird to change them, but if that's important to you that things are themable, as I stated, you can put in the effort to set it up with semantic names and make them customizable at build time or at run time. I still think you'd have a better experience than making separate classes for every tag and reimplementing that for each theme.

> It's not a matter of being afraid to touch HTML. It's a matter of not being able to! ...

And all of these things you list are not what I do. The examples that you list have a different set of trade-offs. I'm not embedding other people's code in my GUI, and I don't expect them to take a random chunk of my application and put them into theirs. OpenDoc was a fun idea, but it didn't take off.

> What made the web as interesting as an application platform was the idea that the user agent could be extended.

I don't really know about that in the general case, I think there are more reasons than that, but in the specific case I'm certain it's wrong. The reason my company is doing what it's doing on the web is 100% about distribution. Arguably, the web stack isn't even that good for making the kind of UIs I do - but if we did what we're doing in Java, we wouldn't get any users (or investors).

I totally agree with you about openness, but openness in my space means providing good APIs and integrations with other services, not allowing user stylesheets. A user stylesheet for my app would mean a twenty page list of weirdly interdependent CSS selectors, and the end result for someone who implemented it would be something like a colour theme that only works for app. Explain to me who would want that.