Chief Justice William H. Rehnquist, 70, sent shock waves through the marble hallways of the U.S. Supreme Court when he showed up wearing a black robe with brilliant gold stripes on each sleeve. For generations, Supreme Court justices have worn plain black robes--a visible sign of their fierce neutrality. Rehnquist said he personally designed the new adornment--four inch-wide horizontal stripes per sleeve, situated midway between shoulder and elbow. He got the idea, he said, by watching the Gilbert & Sullivan comic opera "Iolanthe." A British lord chancellor is decorated that way.