Once color-adjusted, OpenGEM [1, 2] looks hot in high-res. Visually, certainly one of the most beautiful GUIs. The rest of the gems are not (so) agreeable.
These days we've got a full open source stack of the whole ST OS, from the BIOS up to GEM. Including variants that offer Unix-like multitasking with protected memory, etc.
https://os-projects.eu/sites/default/files/2018-05/freemint....