Need logic help

My site has 5 categories.

Each category has a different art scheme. The colors and artwork changes which I change through a function in my page class.

Each Index page for each category looks the same but has the color theme of that category and the links of that category.

Each category also has what I call a content page. Which is basically the content when the user clicks a link in that category.

How would I set this up as class files? Do I create an Idex class and a Content class? Just create one class and extend it? Create an individual class for each categories index and content page?

Any suggestions or help would be greatly appreicated. I kind of feel like a dog chasing his tail.

The way I do it is I make one css file and uses classes to change the different color / layout schema's on the site ..

If the layout is the same it is even easier since you know the $cat you can use this to set the class everywhere and thus change the look for the new cat etc etc ...