The switches used are Kailh Choc Mini PG1232 Low Profile Switches
Since they are in use in actual keyboards, keycap designs already exist in Thingiverse, e.g. this one, and this one
So if you want to 3d-print your own keycaps, the hardest part–designing a stem that actually fits–is already out of the way. All you have to do is correct the dimensions of the upper part, where you don’t have to be as exact.
When all you want to do is color-code the 3 synth buttons and the shift button, you don’t even need to add text to it.
Somebody added a wish for that: