i noticed that if you copy all the css form the ecommerce module to your theme folder, silverstripe will use your versions not the default versions - good for theming...

however, both the checkout and the cart were not picking up the style sheets in my theme folder

I realised that you have to edit ecommerce/code/CartPage.php + ecommerce/code/CheckoutPage.php and find
"public function init() {" and then move the the line "Requirements::themedCSS('CheckoutPage');" to after "parent::init()";

eg:
parent::init();
Requirements::themedCSS('CheckoutPage');

then it will load your nice customised checkout/cart styles...