This is not globally supported on Xtreme, but can be overridden on a case by case basis with some options, or custom CSS in Main options > Fonts & Custom > Custom CSS Rule box (NEVER edit theme files !!)
The two main things that use the smalltablet (768) and phone (581) thresholds, are the Sidebar and the menus. But there are man other small ones, like Widget area layout, columns etc…
If you have “Plus”, you can change the Threshold for the menu (switch to slide open mobile) to anything you want using the smart menus.
If you tell us what part you are trying to change, and what is the reason for it, we may be able to offer suggestions.