Thanks again. I added the first code below to an existing :root stanza and it is effective. If I add the second code, I get the default colour as well, as you say. I hadn't noticed that until I changed the hover colour to a lighter blue.
I do want the colour to apply to menus inside panel menus. Bookmark menus have the set hover colour.
--toolbarbutton-hover-background: #83b7fb !important;
toolbarbutton:hover { background-color: #83b7fb !important; }
I get the following error :