Allow desktop mode down to 800px in Scribe Theme


I'm basically done with building this website, but my client still has a concern before I transfer the site from my development server to go live on his domain.

He has sent me two different screenshots from a large monitor showing a lot of white space on the left and right, but more concerning to him was that the display on his (probably older) laptop and on a Macbook Pro (when not at full screen width—not fully maximized browser window) shows tablet mode. Is there anyway to change this so that desktop mode doesn't change to tablet mode until the resolution is below, say 800px? Even if that were to include scrolling to the right to see the rest of the content? Also, will the menu and footer and header display properly when we scroll over to view content?

I would very much like to keep the tablet and mobile modes, just adjust the desktop mode.

Thank you!