Does dark themes lets your focus more than light themes?
For me, using light-themed software for a prolonged period causes my eyes to strain and my head to ache. Sure, they do so to a very little extent, but the uncomfort is often distracting if not tempting me to slack off.
So for me, yes, dark themes help me get in the zone much easier. But of course that has very little to do of the colors scheme itself.
But what absolutely destroys the experience for me is when I have to look something up on the internet and end up on a page that doesn't honor the browser's dark mode. The switch between bright and dark is extremely jarring.
No such luck however with design tools where changing the theme can't be an option.
However, some people are sensitive to light, so the optics are not the only consideration.
As a colorscheme author, though, I can attest that working on a dark background gives me a lot more "range", so to speak, than on a light background, which tends to make "dark" colorschemes much easier to create and more expressive.
For me, the sweat spot is at around 10-20% lightness.