I have a button with the following set of w3.css classes:
<link rel="stylesheet" href="http://www.w3schools.com/lib/w3.css">
<button class="w3-btn w3-lime w3-hover-green w3-round-large w3-border w3-border-green w3-ripple">Button</button>
mybuttonclass
<button class="mybuttonclass">Button</button>
May be you can add a little bit of jquery, which might be a solution:
if ($('button').hasClass('mybuttonclass')) {
$('button').removeClass('mybuttonclass'); //optional according to your theme requirements
$('button').addClass('w3-btn w3-lime w3-hover-green w3-round-large w3-border w3-border-green w3-ripple');
}