The format is a simple monochrome png bitmap/spritesheet, so custom tilesets are fairly easy to do. The original specifications I talked about in my
2015 ad for a pixel artist. (And there was followup
voting on concepts as well as an eventual
introduction to the set we currently have.)
The main issue with creating a tileset is that each bitmap is only usable by one of the primary UI resolutions, of which there are five, thus for
everyone to be able to use a given set, it must be available in five versions! Kacper has done an excellent and meticulous job of recreating each sprite at five different sizes, maintaining their pixel-perfect appearance across different dimensions.
Certainly one other option for hobbyist tilesets is to make one that simply isn't available at more than one resolution, such as picking the most commonly used 1080p (18x18 fonts). Then only a single bitmap is needed.