Hello,
You can control that here:
Template.CSS - Line 2060
#cboxLoadedContent .button, #s5box_register_inner .button {
background: none repeat scroll 0 0 rgba(0, 0, 0, 0);
border: 3px solid #cccccc;
font-size: 0.9em;
font-weight: 400;
margin-bottom: 16px;
margin-top: 12px;
padding: 9px 23px !important;
}