Infuse JS/CSS per Daniel

Infuse lets you edit a web page live — CSS, JavaScript, and text — from a companion editor page.
You open the editor, copy the short code it shows, paste it into the extension, and start changing things. Edits apply instantly, without reloading. What you change is saved locally as a rule tied to that site, so it comes back the next time you visit.
A note on where this project is:

This is experimental software and it still has bugs. I'm publishing it anyway because it works well enough to be useful, and because real use is the only way it improves. Expect updates.
The source code is public, for transparency — anyone can read exactly what the extension does with the pages it touches.

It's free. It may not always be — if it grows into something more complete, there could be a paid version later. Nothing is collected or sent anywhere either way.

Editor: https://infuse-production-d6d4.up.railway.app/
Source: https://github.com/goldeagle-stack/infuse