You have to add a Style to the report.Styles collection, then assign the style name to the report object's Style property:
// create a new style
Style style = new Style();
style.Name = "MyStyle";
style.Fill = new SolidFill(Color.Red);
// add it to report styles
report.Styles.Add(style);
// apply a style to the Text object
(report.FindObject("Text1") as TextObject).Style = "MyStyle";
Comments
You have to add a Style to the report.Styles collection, then assign the style name to the report object's Style property: