I think there are two way we can do this:
One, we can use javascript libraries, such as https://github.com/sandoche/Darkmode.js ( there are many, so pls check themm out).
Second, this is what I have though of. If you look at the editorial theme, there are Multiple Colour Schemes. A few examples of dark mode:
https://getpublii.com/themes/demo/editorial/dark/
https://getpublii.com/themes/demo/editorial/blue/
https://getpublii.com/themes/demo/editorial/biscay/
For this, i need to create a toggle button to switch color ( write a js code).