I'm on Beta and it's still light theme there. The Dev hasn't changed much yet so I guess it's the same there as well.Not aware of any way to change that currently. I've not checked the latest Dev or Beta builds to see if it is changed there yet - maybe someone can try that and comment on this.
However, that's an ongoing complaint. There are SO MANY things with Windows that Microsoft seems to start and just never finishes. The UI is inconsistent, there are tons of things not yet migrated over from control panel, and on and on the list goes.