How do I change the properties of a row?

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 color, background image, padding, etc.)...

... and some that apply to each column in the row. A row could contain multiple columns, and you can control several settings independently for each column.