Mobile Drop-Down Color | WordPress.org

span.ast-icon.icon-arrow {
    color: brown;
}

To add or override CSS: use the “Additional CSS” option in the customizer.  https://codex.projectdmc.org/CSS#Custom_CSS_in_WordPress or at [site]/wp-admin/customize.php if you don’t have that menu option.

Use the Chrome Developer Tools or Firefox Developer Tools to help you see and test changes to your CSS.