Product recommendations are currently served with products that meet these prerequisites:
The product is purchasable...
... and the status is set to "Publish"...
... and the product is set to "In stock"...
... or when the "Allow backorders" indicator is activated...
... or if the product is downloadable
You may override the standard css using the following classes: zl_products, zl_product, zl_thumbnail, zl_link, zl_image, zl_title, zl_price and zl_button.