Can you suggest a plugin or library for the dark theme feature I want to add to my website? (no optimization problem)
you have several options, and i will list them right below, though i really recommend doing it by yourself if you have the time and resources, it will look better and fill much more natural than using a library (that usually just changes the mix-blend-mode in your css.
darkmode.js - I believe that they are the most known one.
nighly.js - they claim to have zero-dependencies, this could help you be more protected from dependencies vulnerabilities.