no clue then. try downloading a different version maybe that particular one had the option not enabled for some reason. I can confirm that the dark theme works so you are not chasing something that doesn’t exist.
Not sure if that’s the reason, but you’re using an Alpha version (see that a after 2020.2.0?).
Which you should never do, unless you want to test new Editor features and give feedback, as Alpha versions can be very unstable. They are not recommended for production use! — Better: Use the latest LTS.
Have you tried using the release version - 2020.2.0f1 - instead?
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I’m currently developing (well, more like experimenting) in said alpha version, so I’ll install the LTS version once I’m done and see if that fixes it.