var tables = document.EnumerateChildrenOfType<Table>();

var border = new Border(1, BorderStyle.Single, new ThemableColor(Colors.Black));

foreach (var table in tables)
{
    table.Borders = new TableBorders(border);

    foreach (var row in table.Rows)
    {
        foreach (var cell in row.Cells)
        {
            cell.Borders = new TableCellBorders(border);
        }
    }
}