Richard,
I use this to custom syle my buttons. Hopefully it'll work for your buttons.
.mktoForm .mktoButtonWrap.mktoSimple .mktoButton {
color:#FFFFFF !important;
//Place property and styles here.
}
I put !important after each style to make sure the browser uses my styles instead of Marketo's.
Corey Bethel
Consultant | Etumos