Look in /admin/main/configuration.tpl find the code below
Code:
{if $cols_count eq "1"}
{assign var="bgcolor" value=""}
{assign var="cols_count" value=""}
{else}
{assign var="bgcolor" value="bgcolor=#F0F0F0"}
{assign var="cols_count" value="1"}
{/if}
change #F0F0F0 to #000000