Bạn chỉ cần thêm đoạn code sau vào file functions.php của theme trang sử dụng là được nhé

add_filter('woocommerce_sale_flash','devvn_woocommerce_sale_flash', 10, 3);

function devvn_woocommerce_sale_flash($text, $post, $product){

    ob_start();

    $sale_price = get_post_meta( $product->get_id(), '_price', true);

    $regular_price = get_post_meta( $product->get_id(), '_regular_price', true);

    if (empty($regular_price) && $product->is_type( 'variable' )){

        $available_variations = $product->get_available_variations();

        $variation_id = $available_variations[0]['variation_id'];

        $variation = new WC_Product_Variation( $variation_id );

        $regular_price = $variation ->regular_price;

        $sale_price = $variation ->sale_price;

    }

    $sale = ceil(( ($regular_price - $sale_price) / $regular_price ) * 100);

    if ( !empty( $regular_price ) && !empty( $sale_price ) && $regular_price > $sale_price ) :

        $R = floor((255*$sale)/100);

        $G = floor((255*(100-$sale))/100);

        $bg_style = 'background:none;background-color: rgb(' . $R . ',' . $G . ',0);';

        echo apply_filters( 'devvn_woocommerce_sale_flash', '<span class="onsale" style="'. $bg_style .'">-' . $sale . '%</span>', $post, $product );

    endif;

    return ob_get_clean();

}