You probably have to edit the css style of the form.
Here's what worked for me (see tonyt.com > Contact tab):
WP Dashboard > Cforms > Styling > Basic CSS Editor
The file you see in the CSS editor will in theory be the form template in use on your site. For me it was sidebar-layout.css
I found the 'FORM Submit button' section and replaced the code with this:
.cf-sb input { background: grey !important; }
p.cf-sb {
padding: 0!important;
margin: 0;
text-align:center;
In my case the background element was key – I added the text-align because I preferred the button centered at the bottom of the form.
Your results may vary, but the solution likely is therein somewhere, or, a bit further down in the cform input.sendbutton section