This help page explains how to customize the overlay-style cart that appears "inside" your page, such as replacing the standard checkout button with a custom button image:
http://www.e-junkie.com/ej/help.custom-cart.htm
E.g., if that's the only customization you want, you'd add these lines after the "function EJEJC_lc(th) { return false; }" line in your page's View Cart code:
function EJEJC_config() {
EJEJC_POSTCALL=true;
}
function EJEJC_shown() {
jQuery("#btnPP").attr("src", "http://yourdomain/yourimage");
}
Regarding the appearance of PayPal's checkout site, we don't have any direct control over that, but it should look the same for Buy Now and Cart checkouts on desktop computers. If your interest was primarily in having a mobile-optimized checkout on smartphones/tablets, at present that only works for Buy Now buttons or, if you're using PayPal Payments Pro, cart checkouts without tax/shipping.
We recently reached out to PayPal about this, and they confirmed that they do not support a mobile checkout experience for shopping cart orders that use tax/shipping calculation (as ours do), and they have no particular plans to do so in the foreseeable future. That being the case, I've asked Development to adjust our cart checkout integration with PayPal Payments Standard, so that orders without tax/shipping will not send any such calculation details to checkout (as our PayPal Payments Pro integration already does), which should then allow mobile checkout for such orders.