Mobile-first styling issues

Fixed the “white space appears above the Navigation footer and the main Navigation links” part.

Leaving this issue as “Needs review” for the “Notice the white “Expand sidebar” banner at the top” part. Not sure yet if xNavigation needs to style the white “Expand sidebar” banner or use a colour defined in the colour schemes.

The “Expand sidebar” banner styling is now dealt with in ✨ Replace the mobile horizontal sidebar with a corner widget Active .

Automatically closed - issue fixed for 2 weeks with no activity.