Extjs fieldLabel Style

Ext.define('Ext.ux.form', {

    extend: 'Ext.form.Panel',

    listeners: {
        'beforeadd': function(){
            if (!field.allowBlank) {
                field.labelClsExtra = 'x-required';
            }
        }
    }

});

You can then style your field label in CSS with an :after pseudo utility:

.x-required:after {
    content: ' *';
    color: red;
    font-weight: bold;
}