Responsive window size header color

Hi,

On my site bouldersgym.com when I make a desktop window smaller it goes through 3 phases. There's the large desktop size,

the "mobile" size,

and then a size in between where the responsive website changes to having a hamburger dropdown menu instead of the full menu, but is bigger than what is considered a mobile size.

This middle size has a different background color (navy blue) vs the transparent white or light blue set for the desktop and mobile menus. I can't figure out how to change the colors on this middle size using the squarespace editor. 

I was wondering if anyone can help me with this? I just want to change the background color of the hamburger menu to a light blue to match the mobile size and I am not sure how to address this with code.

Thanks in advance!!