The spacing and margins are all manually specified (or, in the case of the example, aren't specified at all, so they default to 0). If you cared to do so, this spacing could be manually adjusted.

Under the hood, it's 100% native widgets (i.e., on OSX, the code is actually instantiating an NSButton, adding it to an NSView, set as content on an NSWindow; on Ubuntu, it uses Gtk.Button etc), so the look and feel of the widgets themselves are exactly as the platform dictates.