Well,
I found the same concern solved already there
floating-taskbar-without-autohide
@Aravisian you're right
It works now by adding the following in .../gnome-shell.css
#panelBox { padding-bottom: 0.4em;
padding-left: 0.6em;
padding-right: 0.6em; }
#panel { border-radius: 10px; }