1) Please post a link so I can investigate the problem. Either the CSS link is not correct, or there is other styling on the page creating a conflict, I will need to see the problem to be able to offer a diagnosis and a solution however.
2) You do not need to modify the targets of the Continue shopping, Update or Clear cart buttons. The clear cart button triggers a server behavior on the cart page that empties the cart. The update button triggers a different server behavior that updates the cart if the quantity is changes or the remove checkbox is checked for an item. The Continue Shiopping button uses a session variabel to go back to the last page the Add to Cart button weas pressed on.
The Checkout button will automatically be updated for you when you run the checkout wizard to create the checkout pages.