How to change responsive widths

Hello, I see in main.css that there are 82 @media calls. Is there a simpler way to change the widths of the theme without editing all 82 instances? For instance, I want the left/main sidebar visible on a tablet on landscape mode (around 960px) and the theme currently doesn’t show it until 1200px.

To do this, I’d like to reduce the size of the main/middle content area.