I’m the CSS actual developer of darktable and I’ve just (merged this day) completely reworked CSS to improve all minor possible glitches and have a cleaner CSS. So I’m just quite sure to what’s possible or not on CSS. And I already talk about that with @anon41087856 who had main the big work (Gtk and first release of darktable 3 CSS) last year about that. He even himself create an issue to query the idea to deprecate bauhaus widget and redraw them. The issue is still open: (maybe) deprecate bauhaus controls · Issue #2077 · darktable-org/darktable · GitHub
And yes, cursors are quite font size related. Or to be more precise, the cursor is a part of an all-in-one widget (bauhaus widget) and that the widget who is size-related (so all things inside also but we can only adjust the widget, not the widget content). So here we can change size, padding, margins, colors of all slider and cursor but not only slider or cursor. That’s one of the limits of those widgets.