实现动态加载主题的核心是修改Page对象的Theme属性

Code
void Page_PreInit(Object sender, EventArgs e)
{
if (Request.QueryString["theme"] == null)
{
theme = "Themes1";
}
else
{
theme = Request.QueryString["theme"];
}
Page.Theme = theme;//加载主题
ListItem item = DropDownList1.Items.FindByValue(theme);
if (item != null)
{
item.Selected = true;
}
}