javascript: (() => { document.documentElement.style.filter = "invert()";document.documentElement.style.background = '#fff' })();
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...
So you can run your own Javascript in your own browser.
Locally.
Good luck.