Transparent menus are a KDE 'feature'. What is happening is that something is causing KDE to set this feature on as default. Just go to control center, find the appropriate setting ( I think it's under 'appearance>styles' but I can't remember & I'm on a win system right now) & disable translucent menus.
BTW, did you mean apt-get upgrade, not apt-get update? apt-get update doesn't do anything that could break your system; it merely updates the list of available packages.