Set Class on a Button within a hidden form

[morozco]

Greetings,

I have a form that i do not have access to. It is only accasable from
our vendor who can not change it, to long to go into. I have
setAttribute to get some color to it but it also but color in the imput
text fields. I need to add a class to the button so i can just change
the color on the button and not the fields. I was thinking
AddAttribute but
<script type="text/javascript">
function init() {
if (!document.getElementById) return false;
var g = document.getElementById('MCWSUBMIT');
var v = g.elements[0];
g.addAttribute("class","btn");
}
</script>
but it is not wrking,

Any help would be great,

Thanks,

Michael