This would be less of a problem now that dark/light mode has become a mainstream thing. Both normal users and developers often switch between light and dark, so bugs would be found quickly and solved with high priority.

The reason theming back in the day was often broken is because it was very niche. That one sysadmin using Linux with a satanic theme complaining about some dark-on-dark text won't even get a reply, but a customer's executive running macOS using its built-in automatic dark/light switcher complaining will be the top priority.

It's a shame the implementation went to a boolen dark/light instead of exposing a bunch of semantic color variables like old toolkits used to do, but we can "thank" designers for that one.