Sorry the code i gave you was slightly wrong. Just add the bit in green below
cd="background-color:#000000; width: 550px; height: 170px; overflow-y:auto;"]
The more rounded you want the edges the higher number you put in. eg. 2em or 3 em. Each 1em stands for 1 corner, (from Left to Right) Top Left | Bottom Left | Top Right | Bottom Right