Need help changing color of hamburger menu on mobile site

Hello,

I need help changing the color of the hamburger menu icon to white. It is currently a dark grey but I would like it to be white on the mobile view. I only want the hamburger icon to change to white and that's it.

This is the website: https://www.sullivandesignstudio.com/ 

As you can see, it is a dark grey but I would like it to be white. Can you provide the code necessary to do this?

Thanks!!

This worked, thanks for the help!