Changing interface styles can be a little difficult for people who haven't tried it before, as it generally involves messing around with scripts and other complicated-sounding things. With a bit of programming knowledge and a tool like Greasemonkey, it's really not that difficult, but it can still be a intimidating for beginners.
Stylish is a Firefox add-on that makes the whole task much easier. It allows you to chose user styles and themes from the Userstyles website and install them directly. The styles allow you to change the appearance of websites, like as changing themes and skins for sites such as Google, Facebook and YouTube. When you open a style on the website, you'll see the option Install with Stylish. This opens a window where you can see what websites the style will affect, preview it and even customize it yourself. This editing will take a little technical knowledge.
If you access Stylish via the Tools > Add-ons window, you'll see that the add-on has added a new section. This will allow you to directly visit the Stylish website, write your own style or check if a Stylish update is available. Obviously, writing your own also takes technical know-how, but there's absolutely no obligation to do the creative part yourself. The styles available on the website are many and varied - obviously, some are more user-friendly than others, but if you have a look around, you're sure to find plenty to suit.
Stylish will help you use and create fun user styles, whether you're experienced or not!
Download Stylish Firefox 1.2.4 in Softonic