Dark mode is fashion. If it were about headaches from bright screens, you would turn down the brightness.

And yet the site is in light mode?

Dark mode is a placebo. If your eyes hurt from the screen, then increase ambient light in the room so the amount of light around you is balanced and your irises will stay in same settings - main source of your pain.

If light would be a big problem, you would be going blind during sunny summer day.