A solution I have used is to add another class + remove any inline styling via jQuery.
$(document).ready(function(){
$('form.mktoForm').find('*').andSelf().toggleClass('myoverrideClass').removeAttr("style");
});
Then use the .myoverrideClass to help override any css from the theme.
For preview and debugging purposes you can strip all the classes and style attributes on page load with jQuery also.
Paste the below in the browser console.
$('form.mktoForm').find('*').andSelf().removeClass().addClass('.myoverrideClass').removeAttr("style");
This will strip all classes and inline styling from the marketo form.
But leave the .myoverrideClass active