In your Woocommerce plugin file, you can find a non-minimized copy of the style sheet. Go to
wp-content/plugins/woocommerce/assets/css/woocommerce.less
That will help you find the class you need to modify.
For example, if you're wanting to add a border to the images on the single product page, look in the Product page section, which starts around line 143.
IMPORTANT: changing this css file will not change anything in your site. HOWEVER, if you make changes to this and then rename it woocommerce.css, it will replace the existing file and the changes will show.