how to create css file dynamic from code behind in asp net

Question

hi Friends,
I want to create dynamic CSS for multiple ASP.NET controls , I have created required css but not able to create the css file where I want to store this css.
For eaxample:
I want to host multiple site with different css /template but same code. so I want to design separate css file based on site Id. so the need is that when a user set their css for all controls then we will save it in a separate css file.

Please tell me the way how can i do this.