Hi! Tried looking, but could not find. Doing a Crash Course tutorial, currently reached this point. In this tutorial you are said to enter Project Explorer → Project Settings → Theme: But in my Mendix, Theme tab is not available: Any help would be appreciated :)
The crash course in the academy was made for Mendix 8.18.3 apparently, so some of the things you're doing might not match up with Mendix 9. Since you're probably already quite far into the course it might be worth just try it out in 8.18.3 to see how it works and then continue in Mx9, but if you want all the documentation to work out, it's probably best to actually switch to 8.18.3 fully.