I'm having great difficulty with something that should be very straight forward and it seems a waste that I should be posting

I want to style the button of my NWS form submit button, but I can't as it does not have pre-assigned "class" to it and I do not know how to attach a class, or does anyone know of any other method. FYI, my site does use the same "submit" style for all forms.
Here is the site: http://www.jenniemarshtrust.org
Thanks