How to customize "hamburger icon" on mobile devices

Hello developers, This time I would like to know how to customize hamburger icon (menu navicon of three lines). I just want to change its size and shift its position a little bit through child theme responsive.css file. Child theme responsive.css has been created already. I see .mobile-menu-barwould be related to the icon customization because it was subject to color change (from default gray to black now). My website is http://kenta-kiyomiya.com Best regards, Kenta Kiyomiya