I know it's not an ideal or permanent solution, but I have a trick I use when I visit dark websites. Many devices allow you to invert colors on the display. On my MacBook Pro it's CTRL+ALT+CMD+8 (disabled by default) and on my iPhone I have it set to 3 presses of the lock button (disabled by default, configurable shortcut). I use it when I visit sites with a dark theme, but then immediately go to one with a light design. It's uncomfortable until my eyes adjust, so I invert dark sites to make them light.I desperately need this. Sometimes I have to just shut this down before I want to because my eyes get so tired and blurry in the night... especially on a tablet or iPad.
We will be releasing a dark theme as a permanent solution, but maybe this could help you in the interim.