To clarify, you just want to remove the “border” from the about us page? If so, can you revert the Container Width to its previous value? There’s a refresh icon there if you want to set it back to the default value.
Also, to be sure, can you take a screenshot of the border you’re referring to?
To clarify, do you want all pages and posts to be full width? Or just remove the Off-white “border”.
If you just want to remove the Off-white “border”, you can do so by selecting an identical color for your Body Background in Appearance > Customize > Colors > Body > Background, similar to the Content Background color.