.scss vs .sass.

gistfile1.txt

@mixin linear-gradient($options...) {
  background: -webkit-linear-gradient($details);
  background: -moz-linear-gradient($details);
  background: linear-gradient($details);
}

.foo {
  @include linear-gradient(top, red, green);
}

// OR

@mixin linear-gradient($options...)
  background: -webkit-linear-gradient($details)
  background: -moz-linear-gradient($details)
  background: linear-gradient($details)
  
.foo
  +linear-gradient(top, red, green);