Someone on this mobileread forum suggested I download and install Calibre from the Calibre website. So that's what I did. But now, the dropdown menus in Calibre are gray text on gray background. In other words, I can't see any words.
https://i.imgur.com/n1Ulse7.png
I'm using Linux Mint 18.2 Cinnamon.
Calibre is version 3.46.0.