Ah ok, thanks for clarifying!
From skimming the various R2 tickets, Create LIght and Dark themes prolly seems to be the most plausible one that could be moved later on into R1. There is already a package in HaikuPorts (flatstyle) that has various flat themes, including attempts at light and dark Haiku themes, While the light one works seems to work well enough the dark themes expose many hardcoded colours not just in BeOS apps, but Haiku programs as well; @nhtello made them and their efforts (and some problems found in apps) can be found in this forum topic.
The next most likely one might be the notification center, as that already has a somewhat working implementation from years ago. Unfortunately, I don’t really know much about how notifications are handled in Haiku to provides any other reasons besides that.