There are some helpful posts here and here.
The code below shows how to also remove the default class (which stopped the addClass from applying) and also including a function.
frappe.ui.form.on('Salary Structure Assignment', {
refresh(frm) {
frm.add_custom_button("Salary Structures...", function(){
frappe.set_route("Form", "salary-structure", "")
})
.addClass("btn-warning").css({'color':'black','font-weight': 'bold'})
.removeClass("btn-default");
}
})