FlowDocument flowDoc = new FlowDocument();
Section sec = new Section();

flowDoc.Background = Brushes.LightSlateGray;
flowDoc.ColumnWidth = 2000;
sec.Background = Brushes.DarkMagenta;
sec.Padding = sec.Margin = new Thickness(0);

Paragraph defPar1 = new Paragraph(new Run("Default paragraph."));
Paragraph defPar2 = new Paragraph(new Run("Default paragraph."));
Paragraph defPar3 = new Paragraph(new Run("Default paragraph."));
Paragraph defPar4 = new Paragraph(new Run("Default paragraph."));
defPar1.Background = defPar2.Background = defPar3.Background = defPar4.Background = Brushes.White;

Paragraph marginPar = new Paragraph(new Run("This paragraph has a magin of 50 pixels set, but no padding."));
marginPar.Background = Brushes.LightBlue;
marginPar.Margin = new Thickness(50);
Paragraph paddingPar = new Paragraph(new Run("This paragraph has padding of 50 pixels set, but no margin."));
paddingPar.Background = Brushes.LightCoral;
paddingPar.Padding = new Thickness(50);
Paragraph marginPaddingPar = new Paragraph(new Run("This paragraph has both padding and margin set to 50 pixels."));
marginPaddingPar.Background = Brushes.LightGreen;
marginPaddingPar.Padding = marginPaddingPar.Margin = new Thickness(50);

sec.Blocks.Add(defPar1);
sec.Blocks.Add(defPar2);
sec.Blocks.Add(marginPar);
sec.Blocks.Add(paddingPar);
sec.Blocks.Add(marginPaddingPar);
sec.Blocks.Add(defPar3);
sec.Blocks.Add(defPar4);
flowDoc.Blocks.Add(sec);