I made a proof-of-concept that is actually pretty simple. Love to hear your feedback. If you have Tampermonkey add-in in your browser, visit https://openuserjs.org/scripts/travelninja/steemit-nightmode and click "install" to enable the nightmode.

You can easily set the times when the nightmode starts and when it ends as well as the background color.

Most likely it will need some updates, so please let me know how you like it - or dislike it!

-- edit: please re-install the script as I'm not sure whether the auto-update works and I fixed a number of issues in version 3 and later