I've read this whole thread but I don't think this is possible to do anymore. The only option I have is to edit the site header, at which point I get a popup saying I can change the Design or Color. I can choose Adaptive, Gradient, or Solid. None of them really do what I need to do. If I choose a white logo, it is illegible against a white background, and if I choose a black logo, it is illegible against a black background. The help documentation seems wildly outdated, still referencing 7.0. From what I can tell, Squarespace has been on 7.1 for at least five years.