CSS, to keep Header BUTTON visible on Mobile

Building a site (in v7.1, Fluid), and I see that Squarespace treats ANY mobile device, of any size, as an iPhone, and moves the crucial Header button into the menu/hamburger rather than displaying it as done in Desktop view - even if there is tons of room for the button on a tablet, or even on a phone in landscape view. I'm looking for CSS that will keep the button visible in any device bigger than a phone held vertically, while keeping my logo/title centered; the menu hamburger all the way left; and the Button all the way right, as seen in Desktop. Any ideas?