I’ve always been one for customising how things look, for example, my firefox address/navigation/bookmark toolbars are all in two lines, in order to maximise space. I also like to make things look different from how they are in their original format. For me, it’s mostly because I think something will look “cooler” to me if this font is changed, or if this color is changed.
For others, it’s because they have a hard time seeing sites that have difficult surroundings, for example a site that uses yellow text on a white background. However, I have found a little extension for Firefox that allows you to change things up. Stylish allows you to do just that.
If you’re knowledgable in CSS scripting, then you can go ahead and make your own styles, but if you need some assistance or just don’t have the tools to make your own, you can go ahead and visit Userstyles where you can find styles for your favorite sites.
My favorite one is the Google Tahoma style (which I’ve since adjusted to use the Lucida Sans font which I think looks very sleek especially after seeing it on a couple of the typepad/typekey blogs out there.
If you’re using opera, most of the scripts can be loaded as scripts, and the same goes for users of the greasemonkey extension.