A simple radial gradient has sophistication.
background: -moz-radial-gradient(circle, yellow, green);

This linear gradient goes from corner to corner. Light colors make for a subtle effect.
background: -moz-linear-gradient(top left, white, lightblue)

With a bit more ambition, you can create a striped gradient like this.
background: -moz-linear-gradient(top, red 0%, orange 20%, yellow 80%, violet 100%);