Take a good look around and check how often people do really change computer platform. And you already have so many things that are different that the "same look" is just an excuse. Gnome, KDE, macOS, Windows does not have the same UX in their file explorer which is a basic utility that everyone has to use. Same with connecting to a WiFi and creating a new user account.

So why would you want Discord to be consistent, when you're mostly using the same desktop (or switch between at most two) for hours.

The thing is when HIG were followed instead of everyone trying to create their "brand", everyone knows where the common actions were. You learned the platform and then can use any app. With the new trend, you would only have one computer, but any new app is a new puzzle to figure out.

I don't really have any issues working out how to use modern electron apps, they all follow very simple UX patterns, I find them much easier to use than the average native wxWidgets/qt app. Simple, consistent UI is less about the color scheme and border radius being consistent and about things being simple and well laid out on a higher level.

Two apps can have different CSS while being easy to understand because the core flows and ideas are the same. While many older native apps feel like junk draw UI with crap thrown everywhere and weird app specific quirks and patterns. Even if it all does use native inputs and windows.