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/* Enqueue Simple Line Icons */
add_action( 'wp_enqueue_scripts', 'simple_line_icons' );
function simple_line_icons() {
    wp_enqueue_style( 'simple-line-icons', 'https://cdnjs.cloudflare.com/ajax/libs/simple-line-icons/2.4.1/css/simple-line-icons.css' );
}