Tobias tells us how a proper app theming mechanism is basically impossible in GNOME, and application ecosystems in general.

“Users” want a lot of things, but just because you want something impossible that doesn’t make it possible. In this case, it’s important to be aware of the costs of giving complete visual freedom to “themes”, both in individual app developer effort, and chilling effects on the ecosystem. If given a choice between customization and more, better apps, I’m confident the majority of people would prefer the latter.

Note that Betula is much more open to customization with CSS. It's a literal setting! I don't use it, of course. If I want to change something visually, I just push it upstream. Y'all folks can do the same, of course, but I won't accept everything. So you've got custom CSS. I think it works well.