Inconsistencies in spacing between boxes, as observed in the Message detail view.

Acceptance criteria:

• In detail view, the message header, body and footer are white boxes on gray background. https://zpl.io/b6MZQpy
• 1px spacing in between the boxes. This gives the appearance of borders.
• Boxes have round corners.
• Check surrounding margins between e.g. action bar and workbench and around workbench