A simple radial gradient has sophistication.
background: radial-gradient(circle, yellow, green);
This linear gradient goes from corner to corner. Light colors make for a subtle effect.
background: linear-gradient(from top left, white, lightblue)
With a bit more ambition, you can create a striped gradient like this.
background: linear-gradient(from top, red 0%, orange 20%, yellow 80%, violet 100%);
A repeating gradient gives even more options.
background: repeating-linear-gradient(to top, red, orange, white, yellow, red 30px);