Hello @Georg !

First, you need to define the level of work you want to do with UNA. If you're going to work with the "how it should look better"area then it will be enough to operate with HTML / CSS. If you want to write own apps for UNA - then it would be good to meet PHP / MySQL / Javascript too. The best starting point is the learning of https://www.w3schools.com/ site which has materials for all mentioned tools.