On mobile devices and narrow screen widths, the Extra Theme displays the navigations links in a mobile menu, which can be opened using the mobile menu button (aka the “hamburger”). To change the color of this menu button, you can use the following CSS:

/* Change hamburger background color */
.show-menu-button { background-color: yellow; }

/* Change hamburger foreground (line) color */
.show-menu-button span { background-color: red; }

You can add this to the theme by pasting it into the “Custom CSS” box found in the ePanel at “Extra > Theme Options > General Settings” (at the very bottom).