Kai's Power Tools / Power Goo / Bryce took the custom interface stuff to the most extreme lengths - people genuinely loved it at the time but I think most would be horrified if anything went that far today.

https://mprove.de/script/99/kai/index.html

Ha! I forgot about those. Man I loved playing with Bryce back in the day, what a magical program that was. And yeah the UI was gorgeous for its time. Heck it still looks great even today, tbh.