Just change background-color to suite your taste and replace with the existing code which begins at [start:voting_items] and finishes at [stop:voting_items].
[start:voting_items]
<tr class="[alt_color]">
<td>[if_voted] [item_title]</td>
<td><table style="width: [voting_width_percent]%; height: 20px; border: 0; padding: 0; margin-top: 5px; margin-left: 5px; margin-right: 5px; margin-bottom: 5px; background-color: #ccc;"><tr><td></td></tr></table>[item_votes] vote(s), [voting_width_percent]%</td>
</tr>
[stop:voting_items]