How do I use CSS or PHP to customize WordPress Form Manager Plugin Table?

many ways to do it, not best but easiest is problably javascript:

<script type="text/javascript">      
jQuery('.fm-item-cell-country:contains("USA")').addClass('americanflag');
</script>

from that you edit your css (which I guess you’re more comfortable with),
setting property for each class (.americanflag in that case)

Hope that help a bit