This add-on injects CSS into web pages. The addons.mozilla.org domain disallows this operation, so this add-on will not work properly when it's run on pages in the addons.mozilla.org domain.

What it does

This extension includes:

a background script, "background.js"
a page action

It adds the page action to every tab the user opens. Clicking the page action for a tab applies some CSS using tabs.insertCSS.

Clicking again removes the CSS using tabs.removeCSS.

What it shows

some basic page action functions
how to apply and remove CSS.