I think what you should focus on is changing the media query threshold so that the CSS applied to the screen width having the desired width is applied at much higher screen widths. Maybe right now the media query says (max-width: 700px ) — try changing that to 3000px or something so the contained CSS is applied at all but the very widest screens.
I’m sort of expecting you to come back with “Yeah, I already tried that”, in which case I wish you’d said so straight away because I don’t have any other ideas. I still think it’s the right approach, but it also must be done correctly. If it didn’t work for you, it could be because you missed something in your attempt.
While it’s OK to experiment with core files to see what works, it’s not a sustainable solution. You’ll need to enqueue your own CSS file so it loads after the core files and overrides the defaults. This means copying the entire media query section to your own CSS just to change that one number.
