I added a little .css file " .config/gtk-3.0/gtk.css" copied below. (there's actually a couple approaches I took, the one I'm using here is not commented out).
/* Two different approaches given below
both valid but with slightly different
behaviour
*/
/* This first approach aggressively radiuses
everything, even items within the panels
themselves.
*/
/*.xfce4-panel {
border-bottom-left-radius: 16px;
border-bottom-right-radius: 16px;
border-top-left-radius: 16px;
border-top-right-radius: 16px;
} */
/* This approach is not as aggresive as above.
Will need to add some transparent seperators
on either end for the radius to show.
(16 px for full radius at my current settings)
*/
.xfce4-panel#XfcePanelWindow {
border-radius: 16px;
}
@import 'colors.css';