I feel like we could go beyond that, especially for more app-like experiences. Maybe we want themes that do things like "add specific trim to make editable fields more identifiable." or adding "high contrast" versions of the themes for low-quality screens or low-vision users.

There's no reason a webpage shouldn't be as themable as, say, a GTK or Qt based desktop application.

We should be trying to snatch back styling power from the designers and putting it back on the user-agent's side. Let the page look brutalist until the user has chosen an appropriate theme for their needs rather than railroading them into what someone in Marketing decided looked good.