Even for apps that don't use one of the native toolkits like GTK or Qt, where this has been a solved problem for decades, they should at least respond to a dark/light mode flag if they can. Flatpaks mostly don't.

They don't even have the same cursor as the system a lot of the time. It's especially cool when running display scaling when some apps shrink the cursor to a minuscule size too because of poor system integration.

I don't think there was ever a standardised way to publish whether the system was in light or dark mode until the last few years.