So I added a custom button but needed it to be blue like the primary button. cur_frm.add_custom_button does not have a provision to set your own button class. I was able to do this with a little jQuery after adding the button.
refresh(frm) {
cur_frm.add_custom_button( __('Button Name'),
function(){
//do something when the button is clicked
}
);
//the button selector is the button label separated by %20 (space)
//we then remove the unwanted class and add the desired one
$("button[data-label='Button%20Name']").removeClass("btn-default").addClass("btn-primary");
}