Stop background (container) from shifting

Without seeing the code it’s hard to try to fix. My initial thoughts would be to try using a css reset, because Safari may have a default value set for it’s margins, paddings, borders, etc.

A couple of helpful reads for you. I hope these will help you find your way to a solution. ??

CSS box model
https://css-tricks.com/the-css-box-model/

In this article, the reset is based on Eric Meyers’
https://themeshaper.com/reset-rebuild-wordpress-theme-css-define-layouts/