Okay, I think I have something that might work for you. Each control (I believe) has an Attributes collection that you can programically add HTML attributes to elements. You can do this with the CustomValidator:
CustomValidator1.Attributes.Add("class", "somevalue");
I am not sure this is persisted via each post. So you may have to run this code on each postback. Also, communicating via forum posts is difficult. If you don't give enough information (too much information is bad too) and I don't know your skill level, I get too general and don't answer your question properly.
"We make a living by what we get, we make a life by what we give." --Winston Churchill