sharepoint css

I don't know whether anyone is working with sharepoint here.I am trying to develope a sharepoint based extranet.this website will be used by different companies.these companies have different colour schemes.so, i have to use different css files for different users.I know there is an SPWeb.altearnatecssurl property.But i have seen that , there are 13 css files in the styles directory.If i want to use different css for different users,where to store them and how to call them .if anyone has any suggestion, please help me out.