I usually do my lesson later at night, so a night mode would really be great on the eyes!
If you're using the app version you can download a app like this one: https://play.google.com/store/apps/details?id=com.eyefilter.nightmode.bluelightfilter&hl=en
If your doing it on your PC then you can simply adjust the brightness on your monitor. Some more recent monitors have an Eye Saver Mode or similar.
Some apps will give you the option to switch to white/grey on black/dark grey for nighttime use. It tends to (IME) work better than just changing the illumination, although that helps too. It would be nice to have a night mode on DL. I have no idea how difficult a thing is to program, though. It doesn't seem like it would be a complex thing, but I fully admit to being fairly clueless in this respect.
If you're using Firefox, try Duolingo Dark 2 (Customizable) style from here:
It is not the newest style but it works great. You can customize how dark the background will be. Try something like 20 20 20 for browser and 10 10 10 for Duo background (Duo darker than the browser).
First, you will need to install Stylus addon for the newest Firefox, or Stylish for older versions.