Theme changes tool bars but not the menu bar

I have LibreOffice installations on two machines - both Linux Mint 19 Cinnamon running LibreOffice 6.0.3.2. When I set a select a FireFox theme, one of them works perfectly, but the other shows the theme on the toolbars, but the menu bar is still grey. I can’t explain the difference in behavior. One is a physical machine and the other is a VM but I doubt that has anything to do with it. Any ideas how to get the themes to show on the menu bar would be greatly appreciated.