Largely agreed - my second bullet was about only ever shifting one of text/background. I'm not certain that darkening backgrounds is more readable than lighten

Largely agreed - my second bullet was about only ever shifting one of text/background. I'm not certain that darkening backgrounds is more readable than lightening text, but it wouldn't surprise me. And in a broader design sense, darkening backgrounds is better for the sort of dark room reading Flux helps with, so it seems obviously preferable on that level.