I guess I could use the "SetExtraStyle" property, but I have no Idea of what it expects as values!
After some searching online I found a CSS example...
menucont.SetExtraStyle($"border-width: 2px 2px 2px 0; /* top right bottom left */
border-style: solid dotted inset double;
border-color: #f00 #0f0 #00f #ff0;"$)
This works as expected, and as per the Thread Title, and Now I realize I miss-phrased the thread title.