I was using rainloop and the UI was better. I'm not sure what's wrong in Snappymail exactly. It's super similar and MUCH more lightweight so it seems that, in the process of optimizing, some important details where lost.

Edit: Now that I've looked a bit into it, I think it's just the font. Snappymail uses Deja Vu Sans while Rainloop used Arial. If I change the css in the firefox inspector, it looks oh so much better with Arial, imho. Icons still look worse though...