ahhh OK
Looking at the file its the table background that needs changing in skin1/customer/main/cart_details.tpl look for the line ..
Code:
<TR{if $bg eq ""}{assign var="bg" value="1"} bgcolor="#FF0000"{else}{assign var="bg" value=""} bgcolor="#EEEEEE"{/if}>
the #FF0000 will turn the table background to Red.