@EAMann has a great answer if you want different layouts for different categories. OTOH if you only want to change the CSS to, for example, have a different background color for each category then you’ll find that most themes will include the category slug as a CSS class on the <body> element and/or possibly other HTML elements that are output by the theme. (And even if your theme doesn’t you could always add that functionality yourself.)
For example the TwentyTen theme does exactly that; for the following URL:
Here’s what the <body> element looks like:
<body class="archive category category-category-1">
Knowing the above you could add the following styles to your theme’s style.css file to have different color backgrounds for each page (I’m being very simplistic here of course; your actual CSS would certainly be more sophisticated):
/* style.css */
body.category-category-1 {
background-color: red;
}
body.category-category-2 {
background-color: green;
}
body.category-category-3 {
background-color: blue;
}
All you really need to do do figure out if your theme does this or not is to view source on the category pages of interest and search for your category slug.
Use a Child Theme
Of course you probably don’t want to modify your theme’s style.css; you probably want to create a Child Theme instead. Here are some answers where I talk about Child Themes:
- Customizing a WordPress Theme without Changing it?
- Making the Header on the Twenty Ten Theme Less Tall?
Note that not all themes support child themes, Thesis being one notable example. In the case of those themes you’ll have to research more into how they are extended.