> Application theming

Software 30 years ago was more amenable to theming. The more system widgets you use, the more effective theming works by swapping them.

Now, we have grudging dark-mode toggles that aren't consistent or universal, not even rising to the level of configurabilty you got with Windows 3.1 themes, let alone things like libXaw3d or libneXtaw where the fundamental widget-drawing code could be swapped out silently.

I get the impression that since about 2005, theming has been on the downturn. Windows XP and OSX both were very close to having first class, user-facing theming systems, but both sort of chickened out at the last minute, and ever since, we've seen less and less control every release.

I think what you're describing as "theming" is more "custom UI". It used to be reserved for games, where stock Windows widgets broke immersion in a medieval fantasy strategy simulator and you were legally obliged to make the cursor a gauntlet or sword. But Electron said to the entire world "go to town, burn the system Human Interface Guidelines and make a branded nightmare!" when your application is a smart-bulb controller or a text editor that could perfectly well fit with native widgets.