Overriding a few CSS styles to enable the float:
.mktoForm {
width: 960px;
}
.mktoForm style + .mktoFormRow {
float: left;
}
.mktoForm style + .mktoFormRow + .mktoFormRow {
clear: right;
}
Restyling forms means a significant CSS investment. You may want to look at my
destyled embedded form example because at a certain point trying to cooperate with Marketo's styles while deeply customizing the form is a losing battle. You're better off just using their base HTML and then creating the look-and-feel from scratch.