I did exactly that for my clock. You could use two fonts with different weights and print them on top of each other. I followed Gordon's suggestion and used images for individual digits. I wrote a very primitive python script to modify fonts, you can have it if you like.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
I did exactly that for my clock. You could use two fonts with different weights and print them on top of each other. I followed Gordon's suggestion and used images for individual digits. I wrote a very primitive python script to modify fonts, you can have it if you like.