WooCommerce: Add to cart shortcode style

I implemented the add_to_cart shortcode of WooCommerce, but default style appears and I don't succeed to modify it.

Shortcode : [add_to_cart id="99"]

Default style defined by woocommerce

array(
  'id' => '99',
  'style' => 'border:4px solid #ccc; padding: 12px;',
  'sku' => 'FOO')

How can I modify this default styles?

(Modifying the main style.css doesn't work)