I changed it in mine....
In includes/templates/YOUR-TEMPLATE/template/tpl_modules_products_quantity_discounts.php
(if that file isnt there, it will be in the default templates folder...)
After: I used this:
Code:
<table width="100%" border="1" bordercolorlight="#CCCCCC" bordercolordark="#CCCCCC" style="border-collapse: collapse" cellspacing="2" cellpadding="2">
Just change the settings to suit your needs....
Of course there probably is an easier CSS way to do this, but it is a quick and easy fix if it isnt already in your CSS File.
Hope this helps.