MIGXdb - add class to input field

I need to customize the layout of a MIGXdb CMP... namely I need to float some of the smaller elements, like check boxes, next to some text boxes.

I know I can add a custom CSS file, but there is no solid way of targeting specific form elements (ids change every time a window spawns), but if there was a way to define a class to an element that would give me a selector to point my css at.

Is there anyway to do this in the Add Field window of MIGXdb or what is the best way to add css to a specific set of elements (not globally in a window)?

With the help of Firebug you'll find that the input element of type="checkbox" has class="x-form-checkbox x-form-field" and is wrapped in a div element having
class="x-form-check-wrap". Those classes are definded in the files index.css and xtheme-modx.css, both in the directory manager/templates/default/css/.
(I'm not sure if this is helpful.)