Database database = HostApplicationServices.WorkingDatabase;
using (Transaction transaction = database.TransactionManager.StartTransaction())
{
ObjectIdCollection vports = database.GetViewports(true);
foreach (ObjectId id in vports)
{
DBDictionary dict = (DBDictionary)transaction.GetObject(database.VisualStyleDictionaryId, OpenMode.ForRead);
Viewport myvp = (Viewport)id.GetObject(OpenMode.ForWrite);
myvp.VisualStyleId = dict.GetAt("2dwireframe");
}
transaction.Commit();
}