Stylish works on Firefox3.0 and higher.Click here to install Stylish Firefox Addon.
Once installed, you can go straight to the Stylish website and install Styles and themes for your favorite websites. These Styles and addons can also be installed via GreaseMonekey addon. A small “S” icon will appear on the bottom right corner of your Firefox browser from where you can control the Stylish Addon and enable/disable various themes and styles.
As soon as the style/theme is installed, browse your favorite website and you can see the website restyled with the addon.