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';

I should have mentioned, the background image is from the finale of Loki. In Norse mythology, it's the Yggdrasil tree.

I like this overall setup for Ultra-Wide monitors. The icon set is Kora-Grey (part of the whole Kora icon pack on gnome-look). The overall theme is Material-Black-Colors (using the Pistachio-BE option here), also on gnome-look. To do the Date - Time display they way I have, I put the clock widget on twice. Once showing only the date, the other showing only the time. Clicking the date brings up Thunderbird open to the Calendar tab. Clicking the time open Thunderbird on the Email tab. To make the panels rounded, I did add a small .css file in the gtk-3 folder. Can show you what I did if you're interested.