Hello,
To change these please reference the following. NOTE: it will change them site wide for all buttons too.
The gray is set here:
.inputbox, .registration input, .login input, .contact-form input, #jform_contact_message, input, textarea, .validate {
background: #ffffff none repeat scroll 0 0;
border: 1px solid #dadada;
color: #848484 !important;
font-size: 0.9em;
font-weight: 300;
padding: 13px 15px !important;
}
template.css line 124
Add this to the bottom of your custom.css file:
#s5_body_padding a.btn {
color: #fff !important;
}