CSS interference

Hi, I’m having a bit of an issue with the CSS of this plugin…simply put, I can’t seem to edit the Add to Cart button at all. Here is the address:
http://www.lavendersdayspas.com/dermalogica/products/exfoliants/skin-prep-scrub/

Any CSS I write for wp_cart_button is simply ignored, but anything for wp_cart_checkout_button isn’t. In addition, it seems like the theme CSS for that particular page is being interfered with, as well: if you look at the accordion-style navigation to the right, the links are all basic links rather than styled, whereas other pages are normal. Help?

http://wordpress.org/extend/plugins/wordpress-simple-paypal-shopping-cart/