can anyone tell me how can i apply css tag to ruby codes
I'm not sure that means anything at all. If you want to style the pages you produce with rails using css then - either the tags themselves are just in the .erb files: do as you usually do - if they are generated by a rails helper (eg form_for, link_to), these all take a hash of html options which you can use to set their class and so on (or inline styles if you are so inclined)
Fred