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.