Unfortunately finding the last .class is not possible with last-of-type.

Edit: To actually add something constructive to this answer, you could fallback to JavaScript to locate this selector, or revise your markup/CSS. Whilst I have found myself in the situation that last-of-type for a class selector would be useful, it's nothing that cannot be worked around with a little more thought.

Make the nomargin a css class, and you can change it easily:

var items = document.getElementsByClassName('nomargin');
items[items.length-1].style.color = 'black';
items[items.length-1].style.background = 'white';

or if that doesn't have enough jQuery, you can use:

jQuery('div.nomargin:last').css({
color:'black',
background:'white'})