I wasn't assuming a toolkit, but I didn't think to call out the differences, since the OP was asking about improving the display of Customize under macOS (which I used extensively until around January of last year). That same context is why I brought up xwidgets: because nswxidget exists, but isn't really stable.
As it turns out, the OP was more concerned about the "icons" that are probably better handled with fonts (similar to all-the-icons, org-bullets, and prettify-symbols-mode) than the buttons and dropdown-menus I (mistakenly) assumed.
While we're on the topic of portability, though: how (how well?) does the support for older Windows versions handle these sort of fonts? It's been decades since I had to look at code points, and that was mostly in the context of European language translations rather than "icons" or emoji. In particular, if the terminal doesn't support such fonts, does it degrade reasonably? Empty rectangles? Boxed numbers?
Thanks in advance,
~Chad