[red_admiral]

The thing here is - around the era of GTK2, when themes (and widgets) were mostly bitmaps, there were a few things you couldn't do that adwaita is great at, but changing your accent color? Hell yes, just set the accent color in a .rc file and you're good, because that just changes the color at a particular index in the palette of the bitmaps used to draw a window.

Earlier still, there was a lot you could do with `~/.Xresources`.

Theming back then was much more a negotiation between "designers" and "users": if I like someone's general style of buttons but I'd like them in red instead of blue, I can easily do that, no coding required. You didn't even have to edit the bitmaps by hand most of the time. Want the widgets from this theme but the window decorations from that theme, but with a different font? Config file, or a few clicks in the settings GUI.

The code of the window manager/desktop environment was written to make this kind of thing possible.

In theory, you could develop a CSS-based theming where you can still factor out some key settings like "accent color" or "roundedness of corners" into variables that are easy to set or edit. Then the OS/DE would ship a default theme that works well, and if the user wants to change those settings to something that works better for them, it's on them not to mess it up.

It seems to me though that adwaita is doing everything short of DRM to make it maximally hard to disagree with their design choices.