Skins available to a control are contained in one or more skin files in a theme directory. The SkinID property specifies which of these skins to apply to the control. A skin is specific to a particular control; you cannot share skin setting between controls of different types.
If you do not set the SkinID property, a control uses the default skin if one is defined. For example, if a skin without an ID is defined for an Image control, then that skin applies to all Image controls that do not explicitly reference a skin by ID and that are not set to disable theming. If a skin with an ID is defined for an Image control, then that skin applies to only Image controls whose SkinID is set to that ID.
If the skin files in a theme directory do not contain a skin with the specified SkinID, an ArgumentException exception is thrown at runtime.