Changing mobile menu color?

I’m using a simple Custom Menu widget in the left sidebar for desktop site navigation of my blog, which has no header content above the article area. In tablet and phone views the sidebar disappears and a large black menu hamburger icon floats up on the upper right in the header area, and clicking the icon opens a slide-out menu panel from the left side of the screen.

I’d like to reverse the colors on the mobile menu hamburger and the slide-out panel (i.e. make the background white, the lines and text black) so it’s less obtrusive against my site’s white background, but none of the GeneratePress Premium customization controls seem to affect this mobile menu. Do I need to do this in CSS?

Any suggestions?

(I’d also like to make the hamburger menu square much smaller (it seems huge on the phone screen), but on thing at a time!)

Thanks, Hal