Toolbar on mobile has transparent background

[hedgefield]

When using v2.0 on mobile, the background becomes transparent at the point where the buttons end.

On desktop that makes sense (and looks good), but on mobile the bar extends across the full width, causing block content below it to show through. So it would need a white background (or fully copy the styling of the desktop version of the toolbar).

[karmatosed]

This will be fixed when we change the toolbar in #4077. For now, lets close this and focus there.