Menus background-color should differ from the actionmenu background-color

I tried to use a LESSCSS `if` but I couldn't get it to work. Instead, the guarded mixin syntax solved the issue.
Note that when this needs to be migrated to native CSS, we could use a CSS variable (or even a media query if we actually use them in other places in the skin ) to make this "isColorThemeDark" condition.