How to select stylesheet by url

I am using vs2010.

My default  page is referenced as follows during testing:
http://localhost:xxxx/default.aspx?/products=a

http://localhost:xxxx/default.aspx?/products=b

I have created  2 styesheets, css1 for product a,  css2 for products b

I want to reference css1 for product a and css2 for product b

The stylesheets are assigned in default.aspx

How can I select a stylesheet by url (example: http://localhost:xxxx/default.aspx?/products=a to use css1) for the body of default.aspx

Thanks in advance,
Isaac