How to add css files based on breakpoint in library?
Here is the solution I found. 

mytheme.libraries.yml

mylibrary:
  version: 1.0
  css:
    theme:
      css/some/path/desktop-small.min.css: { media: all and (min-width: 756px) }
      css/some/path/mobile.min.css: { media: all and (max-width: 755px) }