Override WooCommerce "Sale!" text

functions.php

<?php
/*
 * This function assumes you have a customizer setting of the type 'text-field' setup ready for your custom input
 * If not then it will return the default that can be changed manually @line 11
 */
 
function themeslug_woocommerce_sale_flash( $text ) {
	if ( get_theme_mod( 'themeslug_onsale_text' ) !='' ) {
	$onsale = get_theme_mod( 'themeslug_onsale_text' );
	} else {
		$onsale = __( 'Special Offer!', 'themeslug' );
	}
	$text = '<span class="onsale">'. __( $onsale, 'themeslug' ). '</span>';
	return $text;
}
add_filter( 'woocommerce_sale_flash', 'themeslug_woocommerce_sale_flash' );