Layout changes for different pages (such as ‘About’)

Out of interest, is there an independent php file for the pages? When a visitor clicks on ‘About’ (say) I want the blogg to read a seperate style sheet.
What I want to do is get the theme to call another style sheet that has the side bar ‘spreadout’ as a row along the bottom of the page, rather than a column to one side. I’m also considering the option of having alternate double and/or triple backgrounds for some catagories.

Can anyone give me some hints?

Twilight