I was bored one day and decided to make my gksu(do) and policykit dialogs red. The results are actually quite nice.

Add this to the bottom of your gtkrc file:

style "gksu" {
    bg[NORMAL] = "#770000"
    bg[ACTIVE] = "#550000"
    bg[PRELIGHT] = "#990000"
    bg[SELECTED] = "#550000"
    bg[INSENSITIVE] = "#220000"
}

widget "GksuuiDialog*" style "gksu"widget "PolkitGnomeAuthenticationDialog*" style "gksu"