I'm getting a early start on this issue. I think the Panel color should be gray. The dark green/black color assumes a dark background, which I probably will never choose. It's one thing to have a dark Panel that's easy on the eyes, but it's quite another when Panel is so dark as to be illegible. Moreover, a dark Panel looks incongruously ugly on a lighter background. The standard edition of Mint Daryna (Gnome) has a gray background, and I think Xfce should too. If you get eye strain from looking at a computer screen, maybe you could dim the monitor, or turn off your computer occasionally and get some rest.