change style sheet and other required web them like images

Hi...
I have designed and developed a website in asp.net using visual studio 2010 now I want to change them using admin panel. means i want to design few thems and place these in admin panel and want to give facility to administrator to change them according his choice. now i want to know  that how can i call css stylesheet in masterpage of my user interface using asp.net c#.
can any body help please.
Thanks