Blank Menus in macOS Mojave

If you are receiving blank menus when running Postbox in macOS Mojave, like this:

then you could have modified a default macOS display preference. 

To address this, open a Terminal window in macOS, then type in the following, pressing the Enter key after each line:

defaults delete -g NSRequiresAquaSystemAppearance
killall Dock 
killall Finder