[md8z]

You can do that, but I don't know if I'd recommend it. You'd have to think about what your requirements for the application are. GTK3 is currently stable but will not gain any new features, and all progress is happening in GTK4. So if you don't need any of the new features then you could use GTK3.

Personally I don't think the look needs to be a major factor in your decision though. If you wanted some new features from GTK4 and wanted it to "look at home", and you really wanted to not use any GNOME APIs, then you could avoid using libadwaita and apply the theme yourself. Libadwaita is only useful if you want to follow the current GNOME HIG. I think if MATE/Xfce want to upgrade from GTK3 they would have to do this eventually anyway, and it would actually probably provide a better experience than GTK3 themes did. Also, GTK never had a HIG. You're thinking of the GNOME HIG, and the GNOME 2 HIG is pretty out dated. I think even MATE/Xfce would want to change some of those things in there that we know don't work well with the benefit of 15-20 years of hindsight.