I know that WooCommerce is an out of the box plugin, and that it has no dependency on any of the WooThemes but if the WooCommerce front-end styles are enabled, the css overrides the standard button css (using Canvas).

The following removes the button image that is defined by Canvas, and other WooThemes.

woocommerce.css:49 a.button, button.button, input.button, #review_form #submit {color: white;background: #ff9c07;...}

Using background-color instead of background prevents it overriding any of the WooThemes CSS.