I assume that you already have your css and html.erb files.
when you make your first app using ruby on rails then you want it to look good especially when you know how the magic of css works and then end up using html tags for css stylesheet. but it dont work at all.
when i was getting started with ruby on rails it was a big deal for me to find a way to add custom css to my UI so here are two simple steps and tadda your css start working.
1.You need to put all your css files in assets ->stylesheets
and use <%= stylesheet_link_tag %>