https://ruby-china.org/topics/25217
因为所有scss文件最后都会被application.scss打包成一个css文件
config.assets.precompile += ['nav.css', 'nav_m.css']
<%= stylesheet_link_tag "nav", media: "(min-width: 992px)" %>
<%= stylesheet_link_tag "nav_m", media: "(max-width: 992px)" %>