I remember making a couple map styles when TileMill was a Mapbox product and connected to their APIs. It's now a bit more work set up data for TileMill, but I remember basic styling, such as changing colors and changing the zoom levels at which elements are visible, being fairly easy. Things like symbols for POIs and road number shields are more complicated, but you can get quite far without those. I'm not quite a designer, but definitely interested in map styling. However, I'm more likely to try to get my own online tileserver running and style that with the Mapbox GL JSON style format.