Nord Light was also pretty good when I tried it. I waffle back and forth between light and dark themes now and then and there's always a few good options that brighten the space without flashbanging you.
Now I'm wondering if historically people started writing on blackboards first or in paper. Etching into wood/stone doesn't count since it lacks contrast.
If you look at the Lascaux cave paintings, there's great potential for a soft earthy color theme imho. Black and red on off-white, similar colors as medieval vellum with black ink and vermillion red/orange.