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.