Your right , SkinID, my bad.
If you go into designer view, and click on your control, you can access the properties of that control. Just like a regular control such as textbox.
In the list of properties, you'll see SkinID. Type in the name or ID of your SkinID and set the value.
In your Control code, you can grab the SkinID value. In vb, It's [SkinID] with the square brackets. I don't know what it is in CSharp off the top of my head.
myTextBox.SkinID = [SkinID] 'for VB
myTextBox.CssStyle = [CssStyle]
This is how you can grab predefined values from the property box, and use them in your control.
This is just help on the Control Side issue of your post. I'm not offering help on the skin side issue of your post. I don't do skins.
I've messed around with skins, but ended up using themes. The themes were merely CSS Style collections that could be changed on the fly through code. The skins were system wide in which all Label, TextBox, Button, etc would acquire that look and feel. If I made cooler looking sites such as gaming sites, or music sites, I would use skins. I stopped using themes, because of the large amount of time it takes to create good looking themes.