Sometimes I wonder why there isn't more enthusiasm around theming. Chicago95[0] is popular, but I also love how Garuda[0] themes KDE. There's some small websites for downloading themes on various DEs, but most of them are a bit jank and it seems built-in support beyond basic things like accents aren't there.
[0] https://github.com/grassmunk/Chicago95
[1] https://garudalinux.org/editions (screenshots don't do it justice)
I used to really enjoy theming and Riceing, but then I realised it was pointless: my monitor always looks the same, with a full screen IDE window covering up all my fancy themes
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.