I don't want to install complex plugins in the browser, because they get access to my data. I want to build it myself, but I have no idea.
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.
Enable reader mode on your browser. It's built-in, no plugins needed. Open source is the wrong way.