XF 2.1 How to use min-width around entire forum or limit responsive width?

[Feanor]

I'm trying to make a style that doesn't get so small it turns into a "mobile version". Instead, when viewed on a mobile device, it would just look like a desktop version and zoomed out more. Maybe a little responsiveness is okay, but let's say no smaller than 900px. What's the easiest way to do this?

Thanks!