Flexbox column swap without media or container querie.

I watched this video by KP (https://www.youtube.com/watch?v=LEkFckg7sfw), and got the flex solution to work. I'm using a grid to control the logo and the image in the spirit of Jen Simmons (avoiding any queries), which is placed inside the h1, and the nav sits in the second column, and this is the content that pops underneath. I've been back and forth, and have gone a bit number blind, but I'm not able to control the nav-size, and to make the pop to one column at a smaller width. I'm changing the --_main-min-width & --_sidebar-size, but nothing is happening. Is there some issues with how the grid in the h1 is behaving? I'm adding an image of the desktop layout. Here's the repo: https://github.com/AMarlonG/Rondo-prototype