I think that speaks to another aspect, individual apps taking full control over how they're presented instead of inheriting whatever framework the DE is providing or the cohabitation of various KDE/QT/GTK/X/other, or electron framework defaults. Over on windows even when uxtheme skinning was in full swing it was the start of applications doing it themselves (winamp and quicktime come to mind), but I have the impression developers doing so made sure the extra effort had a payoff.

I remember back in the day the apps most likely to have completely custom UIs tended to be scanner drivers. I'm not sure why!