Transparent system panel in Linux Mint 18.x Cinnamon
Cinnamon Menu Transparency Question
Question: Where is the GUI control for disabling menu transparency?
Answer: There is no GUI!
Modify /usr/share/cinnamon/theme/cinnamon.css or use another Theme.
How To Customize Basic Cinnamon Elements
10 things to do first in Linux Mint 19 Cinnamon
Speed up your Mint 19!