I check my Bitcoin Core in the Display tab and see no option for wallet Theme display mode so likely those discussions did not turn into any visual deployment from Bitcoin Core developers.
Yes, there is no option to change dark theme mode like electrum. I lift this with hope the developer make it happen in the future. I know it's not more important, but if it's easy to implement without disturbing other programs, why not?
You can force dark mode on your OS, whether it is a version of Windows or Ubuntu.
I used dark mode on windows, but the bitcoin core wallet still did not follow the system. I don't know in Linux, but in Android, all the applications will follow the phone theme when we click application to follow the system.
Bitcoin Core makes use of Qt for windowing and as far as I know it doesn't have an automatic dark mode. You have to style it that way yourself. Given that there are a lot of screens, it's not surprising to me that this is taking so long as it's a log of work.
You are right, maybe I have to test and follow that GitHub code, or maybe I have to use it to another PC so it's safer rather than testing it on a wallet that has a balance.