The RadSkinManager has some missing controls(e.g. RadCard) in the ControlTypeToApplySkin definition, hence the following approach does not work for it:
skinmanager.TargetControls.Add(ControlTypeToApplySkin.RadCard ,"Vista"); //ControlTypeToApplySkin.RadCard does not exist
When loading custom skin trough RadSkinManager -> Skins -> SkinReference.Path, the logic inside RadSkinManager.FillNonEmbeddedSkinsInfo does not work correctly: in order to load ComboBox.MySkin.css, per say, it requires ComboBox folder to be present inside the custom skin folder.