What you are looking for is documented on: https://github.com/yeoman/grunt-usemin
Simply wrap your css imports in a comment block similarly to the way it’s done with the javascript files
<!-- build:css styles/main.css -->
<link rel="stylesheet" href="https://stackoverflow.com/questions/15617431/styles/base.css">
<link rel="stylesheet" href="styles/modules.css">
<link rel="stylesheet" href="styles/layout.css">
<!-- endbuild -->
make sure your generator is up to date and your grunt tasks are all set. The one doing the magic is 'useminPrepare'