In your RMQ application, you should have an ApplicationStylesheet class from which all your other stylesheets should inherit.
You could add a default_label method in this ApplicationStylesheet:
def default_label(st)
st.background_color = color.light_gray
end
To apply the style you would have to use it when you append your label
rmq.append UILabel, :default_label
Same goes for your AttributeUILabel, create a method in your ApplicationStylesheet and use the style when you append it to the view.
I suggest you go back to the RMQ stylesheets documentation, it shows everything you need to know about styling.