可以设置Webpart的System.Web.UI.Control.IsThemeEnabled属性
The EnableTheming property indicates whether themes are enabled for the PlaceHolder control. The EnableTheming property also affects whether themes are enabled for the controls contained in its Control.Controls collection. When the EnableTheming property is true, the application's theme directory is searched for control skins to apply. If no skin for the particular control exists in the theme directory, a skin is not applied.
具体请参照
http://msdn2.microsoft.com/en-us/library/system.web.ui.webcontrols.placeholder.enabletheming(VS.80).aspx
