Thank you, I figured it out. For anyone else who is trying to do this, it was in the /customer/main/cart_details.tpl under this area:
<TR{if $bg eq ""}{assign var="bg" value="1"} bgcolor="#A52A2A"{else}{assign var="bg" value=""} bgcolor="#A52A2A"{/if}>
<TD>
{if $products[prod_num].productcode}{$products[prod_num].productcode}{else}#{$products[prod_num].productid}{/if}. {$products[prod_num].product|truncate:"30":"...":true}</TD>
{if $cart.display_cart_products_tax_rates eq "Y"}
<TD align="center">
{foreach from=$products[prod_num].taxes key=tax_name item=tax}
{if $tax.tax_value gt 0}
{if $cart.product_tax_name eq ""}<NOBR>{$tax.tax_display_name}</NOBR> {/if}
{if $tax.rate_type eq "%"}{$tax.rate_value}%{else}{include file="currency.tpl" value=$tax.rate_value}{/if}
{/if}
{/foreach}
This part here:
<TR{if $bg eq ""}{assign var="bg" value="1"} bgcolor="#A52A2A"{else}{assign var="bg" value=""} bgcolor="#A52A2A"{/if}>
has the bgcolor=" place to enter your new background color values.