Enable reader mode on your browser. It's built-in, no plugins needed. Open source is the wrong way.

To endarken any page while you're browsing, just execute this javascript in a bookmark:

javascript: (() => { document.documentElement.style.filter = "invert()";document.documentElement.style.background = '#fff' })();

userContent.css

https://superuser.com/questions/318912/how-can-i-override-th...

You need to enable it in about:config these days

https://www.userchrome.org/firefox-changes-userchrome-css.ht...

User scripts using TamperMonkey.

So you can run your own Javascript in your own browser.

Locally.

Good luck.