That is controlled by the cleartable class
<tr>
<td colspan="4" align="right" class="cleartable">
The code is in the dosearch.tmpl However this template has not changed really from 6.3 to 6.4 It used the same class in that version.
.cleartable {
background: #FFFFFF;
color: #000000;
}
This is why you see the white background. The problem is that you may see what you think is nasty on some boards is because they use darker stylesheets and probally do not update their cleartable to the color of the page body. Thus it looks out of place.