gistfile1.txt

Add this code block to php file:

// Add Backorder text to woocommerce images that are backordered products
function ff_add_backorder_badge() {
    global $product;

    if($product->stock !== '' && $product->stock <= 0) {
        echo '<span class="out-of-stock">Backorder</span>';
    }
}
add_action( 'woocommerce_before_shop_loop_item_title', 'ff_add_backorder_badge', 9 );


Add this CSS:
/*woocommerce available on backorder badge*/
.woocommerce .out-of-stock {
    position: absolute;
    top: 0;
    right: 0;
    margin: 0;
    background-color: #bcb1b3;
    color: #ffffff;
    z-index: 10;
    padding: 5px 10px;
}