var table = header.Blocks.AddTable();
table.PreferredWidth = new TableWidthUnit(TableWidthUnitType.Percent, 100);
var row = table.Rows.AddTableRow();
var cell = row.Cells.AddTableCell();
cell.Shading.BackgroundColor = new ThemableColor(Colors.Red);

var head = new Run(doc);
head.Text = "Test";
var paragraph = cell.Blocks.AddParagraph();
paragraph.TextAlignment = Telerik.Windows.Documents.Flow.Model.Styles.Alignment.Center;
paragraph.Inlines.Add(head);