Question: the thing I am asking is like this,say I went to webpage but I dont like the font-face they used or the font-size or may be the background color, can I change it from my pc using any local css? In firefox, there is a plugin firebug, using which I can do some changes temporarily, but I want it to remain unless I change it manually,even though just only for my pc. Now, my idea is telling I have to develop a browser plugin.

I want to design a system where user with disability such as vision impairment or color-blindness can modify the site according to their needs. Some good web sites do give this privileges to change font-size or color,but in my case I want to make something universal. Is there any suggestion?

Answer: 
yes, I remember an extension for firefox: [greasemonkey](https://addons.mozilla.org/en-US/firefox/addon/greasemonkey/).

It's easy to change color and other styles of standard elements, like background, headers (h1, ...), paragraph, ... but I don't know about custom css class that are widely used across web sites. So at the end probably it's difficult to write a general script that works for every website. It is based on javascripts, so it is very powerfull
You could use [Greasemonkey](https://addons.mozilla.org/en-US/firefox/addon/greasemonkey/), and associate a Javascript with every page you want to modify.