select background image
Set the checkout "Select Background Image" to tile from the middle instead of from the left. I tried to add a background image 780 pixels white in the middle (where the checkout info is) with gray to the left and right to "border" the checkout page. It looks great, but is only centered at my actual screen resolution of 1900 wide, which is the width of the background image I uploaded.
If the background image was set to tile from the middle, the 780 pixels of white would always be centered over the actual 1SC checkout form, and to the left and right could be whatever background color or pattern was being used to border the checkout page form.
It'd be a super-easy way to make the checkout page look more complete and cohesive, and of course the background color of the left and right side areas could be set to the same color as the merchant's website for continuity. Let's do it! :-)