How to automatically use custom css when loading a certain site? (Firefox)

Question

css

Task:
1. There is a Mozilla Firefox browser and in it: Firebug, Web-developer, Ad-blocks;
2. It is necessary, when entering the site, to change the css of the service to custom ones (more precisely, it is better not to load the css of the site at all).

Question: What is the best way to do this?

Thank you for your attention!

Answer 1

style ?

Answer 2

Today I found Stylebot for Google Chrome . Cool stuff.