I know GWT is similar to jQuery, but I've never used it... with jQuery I'd do this (I wasn't sure what kind of button tag you were using, so I included both):
CSS
input, button {
background-color: #555;
color: #ddd;
}
.clicked {
background-color: #f80;
}
HTML
<button type="button">Click Me</button>
<input type="button" value="Click Me" />
Script
$(document).ready(function(){
$('button, :button')
.mousedown(function(){ $(this).addClass('clicked') })
.mouseup(function(){ $(this).removeClass('clicked') })
.mouseout(function(){ $(this).removeClass('clicked') });
})