How to stretch background:

To stretch background, first click on design mode. Next, click the background tab and choose a zone. Then, hover over the zone and click the pencil icon. Finally, click the 'full background' checkbox.

The 'full background' check box will allow the background to be full screen.

Here is how to stretch background:

Step 1: Click the design tab. 

Step 2: Click Background tab (A). 

Step 3: Choose a zone (B). 

Step 4: Hover over the zone and click the pencil icon (same as editing a widget)

Step 5: Check the box next to 'full background' (C).