The support of Themes within custom controls can be implemented extremely easily. First, you must decide whether each property should be available via a Skin or not. If the control derives from a base class like WebControl, then the decision has already been made with regard to a lot of existing properties. You just have to point the new Themeable attribute to all the other properties you want to allow to be assigned through a Skin. That's all you have to do!
[Themeable]
public string MyProperty
{
get { return this.myProperty; }
set { this.myProperty = value; }
}