First mouse over an area of the stage that is free of message content.
Then, click to select it.
The right side of the editor will now show you some properties that apply to the entire row (e.g. background colour) and some that apply to each column in the row. A row could contain multiple columns.
- Background colour for that column
- Padding settings (for all sides or for each side)
- Border settings (for all sides or for each side)