There are 2 pages on your site that display information in iFrames.
Note - both of these pages only linked to from events where you are ticketing based on reserved spaces (not price groups/sections)
Be default the background color is white, you should change this color to match your site.
Here's an example of the iFrame with a white background:
To change this:
Select the color picker to change the color:
Select a new color for the palette.
Then click the save button.
Then return to your site to view your change.
In this case we now have a black background and it matches our site: