Classic Mac OS window headers had striped or dot patterns that were kind of similar to rugged parts of various physical tool handles. So they were both decorative in a way and informational: this is a part you can hold and drag around. A typical interface will have a lot of such parts that are both functional and decorative, so it will feel just right.

Purely decorative parts are also possible (and even desirable), but they should be personal. A set of colors chosen by the user, a background texture, a picture that the user keeps on the desktop and so on.