If you want to contribute, or make this extension better, you can mainly do four things.
Firstly, you can do monetary donations. Currently, this project is being funded by my personal savings, and I cannot afford to fund this forever, so even small contributions may help. I could have easily made this a paid plugin considering that it is way better than even the paid plugins out there, but I didn't want to block such a useful plugin behind a paywall. That is the reason why it exists as a free opensource plugin. The thing is that, everything costs money and it is difficult to continue development without enough funds. We have not yet setup a way to accept payments yet, but will do that soon.
Secondly, you can contribute to our codebase. You can contribute the native css darkmode of a particular popular site. We will review your submission and will add it to the extension in the next version.
Thirdly, you can share our extension's links with your friends on the social media or recommend it to more people as it will get better and better as more and more people use it.
And finally, you can give us good ratings/reviews in the chrome webstore.