add_filter( 'woocommerce_get_availability', 'dispay_custom_icons_for_availability', 1, 2);
function dispay_custom_icons_for_availability( $availability, $product ) {
   global $product;

   // available
    if ( $product->is_in_stock() ) {
        $availability['availability'] = __('GREEN ICON HERE', 'woocommerce');
    }

    // middle stock
    if ( $product->is_in_stock() && $product->get_stock_quantity() <= 20 ) {
        $availability['availability'] = sprintf( __('ORANGE ICON HERE', 'woocommerce'), $product->get_stock_quantity());
 }


   // out of stock
    if ( ! $product->is_in_stock() ) {
        $availability['availability'] = __('RED ICON HERE', 'woocommerce');
    }

    return $availability;
}