Hi kuanhwa14,

Currently this feature is not possible in Kalium, we are completely redesigning the shop pages and this feature will be included soon, in the meantime if you want to implement it now add this feature now add this code under functions.php of the Kalium - Child Theme:

add_action( 'woocommerce_sale_flash', 'sale_badge_percentage', 25 );
 
function sale_badge_percentage() {
   global $product;
   if ( ! $product->is_on_sale() ) return;
   if ( $product->is_type( 'simple' ) ) {
      $max_percentage = ( ( $product->get_regular_price() - $product->get_sale_price() ) / $product->get_regular_price() ) * 100;
   } elseif ( $product->is_type( 'variable' ) ) {
      $max_percentage = 0;
      foreach ( $product->get_children() as $child_id ) {
         $variation = wc_get_product( $child_id );
         $price = $variation->get_regular_price();
         $sale = $variation->get_sale_price();
         if ( $price != 0 && ! empty( $sale ) ) $percentage = ( $price - $sale ) / $price * 100;
         if ( $percentage > $max_percentage ) {
            $max_percentage = $percentage;
         }
      }
   }
   if ( $max_percentage > 0 ) echo "<span class="onsale">-" . round($max_percentage) . "%</span>"; // If you would like to show -40% off then add text after % sign
}

If you don't know how to add custom code to functions.php file, follow this guide from our documentation.

Sources:

https://wpsimplehacks.com/how-to-display-the-discount-percentage-on-the-sale-badge/

https://divi.space/wordpress-and-divi-code-snippets/change-woocommerce-sale-badge-display-percentage-discount/