If you just create a new form it will use the Form Editor 2.0. To override the CSS, you have two options:
1. In tab #2 (Form Settings > Form Theme), click the gear icon in the top-right side and say "Edit Custom CSS"
Add something like:
.mktoLabel {
font-family:"Verdana";
}
2. Add the snippet below to the <HEAD> portion of the web page that is hosting the form:
<style>
.mktoLabel {
font-family:"Verdana";
}
</style>
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