Thank you very much for your help.
(1) I have no CSS editing experience; so, provided I got the custom design upgrade, could you tell me if removing that line is a complicated task, or a somewhat accessible one, as I'm hoping, even for someone without experience?
(2) I'd read that excellent article by Panos that you refer me to before I posted my question, but I must be missing something or doing something wrong. I've followed this procedure: "to create this complex showcase front, you go to Pages > Add New and create two pages, then go to Settings > Reading and set the one as your posts page and the other as your front page. The first one must be blank. For the second one you must have selected the showcase template in the Page Attributes module of the page editor". My front page has a showcase template and the post page a sidebar one; both are empty. Still, the widgets placed on the post page's side bar don't show up at the lower left of the front one, as I think they should (if I activate the ephemera widget, it does show; but only that one). I've tried switching the page I've created to work as the front one to a sidebar template, instead of the showcase one, but then, obviously, the front page stops working.
I don't know what I'm missing or doing wrong.