http://jekyllrb.com/docs/assets/

Jekyll provides built-in support for Sass and CoffeeScript. In order to use
them, create a file with the proper extension name (one of .sass, .scss,
or .coffee) and start the file with two lines of triple dashes, like this:

---

---

// start content
.my-definition
  font-size: 1.2em

except that front matter (the yaml stuff at the top) is not proper sass; and makes tools like grunt-scss-lint or scss-lint barf.

see also robwierzbowski/generator-jekyllrb#130