Hello,

I've used the following css class definition and it works without problems:

.CUSTOMCSS
{
background-color: #FF00FF; /*pink*/
}

Have you defined the class in the correct css stylesheet? Have you the class defined at all?
What is a repeater menu? This property will be only taken into account if you use our default navigation webparts which are available in Kentico out of the box

Best regards,
Boris Pocatko