cart and checkout pages

Im trying to customize both of these pages (cart & checkout). ive tried using css but they dont seem to change much apart from the colours. I understand that maybe editing the actual php theme files, but again unable to locate the cart.php or checkout.php. Where am i going wrong? If someone could direct me to where or what pages or plugin or whatever is necessary so that i am able to edit some of the tables and layouts of both these page, i would be more than grateful.