Unfortunately, I think this has changed from previous versions. Right clicking on the panel gives me: panel option - panel settings, where I can choose to add/remove widgets, change panel edge and height --
OK - just then I clicked on "height" and as soon as I dragged upwards on the panel, it changed from the default blue to transparent! I then dragged it back down to it's previous size, and now it's staying transparent. I think that's a little weird but hey it works!