Hello again Wouter! Basically the theme is Boost with some settings in the admin area that includes a link to an image that is uploaded online and that forms the background you see on the site. If you watch this video of the Boost theme for administrators, it shows me doing the same thing as on Learn Moodle, pretty much.
There is some documentation on Boost presets
here https://docs.moodle.org/dev/Boost_Presets but I'm sorry I don't understand theming so can't help more.