var query = from t in Themes
                        group t by t.Name into g
                        orderby g.Count() descending
                        select new
                        {
                            Key = g.Key,
                            Count = g.Count(),
                            Name = from t in g
                                   select t.Author
                        };

var query = from t in Themes
                        group t by t.Name into g
                        orderby g.Count() descending
                        select new
                        {
                            Key = g.Key,
                            Count = g.Count(),
                            Author = from t in g
                                   select t.Author
                        };