After trying to remove a wall with my head for the better part of an hour, I finally figured out the source of my frustration.... something with the way checkboxes and labels are coded within e-newsletter. See here:
An e-newsletter subscription box and a regular form with checkbox. The regular form checkbox works perfectly. (Using a typical method of replacing the default checkbox and adding styling to the label:before property.)
Just cannot get this to work with the WPMU form. My only guess is that it has something to do with the way the checkboxes and labels are rendered... that they are on separate lines within the HTML.
So, has anyone been able to successfully restyle their checkboxes used in e-news? Maybe another method that works? Or do I have to try hacking the plugin?