This is what WooCommerce told me:
Sean (WooThemes)
Jul 24, 23:08
Hi Bartosz,
It looks like the theme being used on the site is overriding how PayPal Express displays it’s buttons. However, when I switched the button style it displayed properly: http://cld.wthms.co/1aTOe/z6t45TKR
However, if you want the button to be the other style (WooCommerce-style), then I’d suggest getting in touch with the theme developer to find out why they are overriding the default WooCommerce cart page in such that it’s not letting the PayPal Express buttons display properly.
Sean – WooCommerceNinja
WooThemes Support