The current style of that little table is hard-coded into ...

includes/templates/template_default/templates/tpl_modules_products_quantity_discounts.php

It can be changed there, of course, and the edited file added to the correct overrides folder, but it would be better to remove the hard-coded styles and style the table in the stylesheet. The DIV surrounding that table has a class attached to it, though it's not in the default stylesheet. You can add it like this, down at the bottom, ...

.productQuantityDiscounts {
[ your styles go here ]
}

Rob