it doesn't appear to be in the css.

Code:
http://4horsementackle.com/mthumb.php?src=includes/templates/FZCF7211/images/logo.png&zc=2&w=105px&h=35px
That is the link to logo when in the small viewport. So when the logo.png is processed by the mthumb.php it is the resulting image that ends up being produced that has white sides on it. How to resolve that I do not know as I haven't worked with this template, but that should get you pointed in the right direction.