Hey Barb, when the OP mentioned the bottom and right black lines, I think they mean that the right and bottom edges of the page have a thicker black line than do the top and left edges, which I think are only to meant to imply a small drop shadow that distinguishes the page from the pasteboard. I've never given it any thought, because it's always just been there as far as I can remember. I've never considered turning it off in normal screen mode.
Yes @Migintosh, but I am assuming the question is coming up because it looks different than a normal document with defined margin values. If I missed the point of the question, @defaultu9mhwgm2mqug, please let us know. I do see the faint pink right and bottom margin lines along the shadow, so the margins are set to 0.