This is probably a setting I don’t know about.  I am an extreme neophyte.  But when I try to view the theme “Customize” or “Header” menu items I see only a blank screen.  I believe I could see the options when I worked on it yesterday, but as I was trying out a couple of themes, I can’t be 100% certain.  I’ve logged out and back in, and I’ve tried from two browsers (Safari, Firefox).  Here is a screenshot of what I see on the “header” option:
Screenshot
I’m just trying the theme out, but I like it and am considering the premium version.  Please let me know how I can see the options that should be under those two links.  Thank you!