Thank you so much for your quick reply. The instructions in the link you provided looked a little over my head. So instead, I figured out a workaround that will be fine for me, and will speed up my load time.
1. In JetPack I deactivated the Custom CSS module (deactivate button hidden under the Learn More button).
2. In Appearance > Customize I changed the header color to black (the color needed for the mobile theme). (I had previously had it set to white.)
3. In my child theme style.css file I changed the header color to white (the color needed for my desktop theme).
My website load time seems to be faster already with the Custom CSS deactivated, but I will keep checking the the site speed throughout the day to make sure it stays fast.