Absolute centering.

SassMeister-input-HTML.html

<div id="container">
	<div id="inner">content</div>
</div>

SassMeister-input.scss

// ----
// Sass (v3.3.4)
// Compass (v1.0.0.alpha.18)
// ----

#container {
  background: red;
  width: 300px; 
  height: 100px;
  margin: auto;
  position: fixed;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  overflow-y: scroll;
}

div {
  color: green;
}

SassMeister-output.css

#container {
  background: red;
  width: 300px;
  height: 100px;
  margin: auto;
  position: fixed;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  overflow-y: scroll;
}

div {
  color: green;
}

SassMeister-rendered.html

<div id="container">
	<div id="inner">content</div>
</div>