Oh! I figured this out: There is no shadow on the small menu… normally; I put it there, by implementing the Thirteen Colors plug-in.
This is a neat plug-in, because it lets you tweak the colors, but I learned the hard way that if you tweak this theme in any way you have to test it at all browser widths, because every change needs to play across each of these “versions” of the theme.
And, having deactivated this plug-in, it gives me a renewed respect for the theme’s exact, original colors. I had only tweaked the menu color a tiny bit, and the original is so much better, because of how the colors work together.
Thanks again for this theme!