Hi Horace,
You can apply CSS only for Service Studio preview using the following convention:
-servicestudio-<Css property Name>: <Property Value>
Being that said, you can include in your Theme, a CSS instruction like this to make the menu disappear on Service Studio preview:
.menu {
-servicestudio-display: none;
}
I'm not sure if the class you want to apply the style is menu but this is something easy to check and change.
Hope it helps.
Regards,
João