There are next to no build-in UI customization settings, despite the devs promosing it last year when DE was announced. I think you have to mod it.
iirc, control group icons are a widget in the same file as the bottom left panel. Think it was called command-panel.json (or something with command, can't recall exact name and can't check atm)Whole UI is easily customizable though which is what I think is was said to be. You can change size of hud and tooltips from options.
anyway
what is name of file for control group icons