I'm trying to add custom colors to the WYSISYG blog editor so every time a user opens it they have their brand colors available. It appears the recommendation online is to update the functions.php file but I'm very novice and don't want to break the site, lose it on the next update, or really see where to update it. The only option available to me is in the Theme File Editor (screenshot attached) but this seems to be pulling coding from somewhere else, and I'd prefer to do it in a child file but don't how to do that. Can someone provide guidance on a plugin to use or where exactly to add the custom coding? 

function mytheme_setup_theme_supported_features() { add_theme_support( 'editor-color-palette', array( array( 'name' => __( 'dark blue', 'yourthemename' ), 'slug' => 'dark-blue', 'color' => '#2193b0', ), array( 'name' => __( 'light blue', 'yourthemename' ), 'slug' => 'light-blue', 'color' => '#6dd5ed', ), array( 'name' => __( 'orange', 'yourthemename' ), 'slug' => 'orange', 'color' => '#ff9900', ), array( 'name' => __( 'green', 'yourthemename' ), 'slug' => 'green', 'color' => '#8aa346', ), ) );
} add_action( 'after_setup_theme', 'mytheme_setup_theme_supported_features' );