I was also a bit confused as to why the title was "DarkMode Made Easy", especially since your example just showed adjusting CSS variables. I would assume anyone who's working with CSS variables, also knows about media queries, so it's far easier to just do it all in CSS.
Where I could see this being useful is that you have a callback that you can use, so when dark mode is enabled, you can make some other adjustments when needed.
You'll have a callback on the next version. I plan a lot of tweaks and features that can hardly be done only in CSS, moreover the plan is to give devs an easy way to do dark-modes. I understand the divergences and critics but want to try things as a student. I thanks you a lot for your review.
I honestly recommend writing your own. The code is presently close to trivial, and if you write such a thing yourself you can take it in the direction that suits your requirements.
Where I could see this being useful is that you have a callback that you can use, so when dark mode is enabled, you can make some other adjustments when needed.