The width of the site (relative to the screen width) is automatically determined by either of 2 settings:

  1. Site-wide background color
  2. Site-wide background image

If site-wide background color is set to #ffffff (white), and no site-wide background image is set, the site will span the full width of the screen. Otherwise, it would be centered on the screen with space to the left and right of it.

Both settings can be found in the customizer:

  • For site-wide background color, go to ‘Colors‘ -> ‘General‘ section.
  • For site-wide background image, go to ‘Background Image‘ section.