provide code behind for ur master page and load the css file to literal control which you will place in master page

Instead of writing code behind for Master Page and then having a PAGE PARSER Path, you can create a simple user control that will identify the user and his group either SPUserProfile property or what ever logic you want and then change the CSS file or class accordingly, that should change the appearance of PAGES.

Can someone give me any link which will help me.

This link will help you get started but this is not the solution to your problem, basically this will help you understanding adding a user control to the Master Page (that will generate the link for CSS (custom file, that is user dependant) in the Master Page.

www.helloitsliam.com/.../sharepoint-%E2%80%93-user-control-in-master-page-page-footer.aspx