I can not find any thing that allows a user to set page margins. Instead there is a little hack to do that by using tables. Go to Insert > Table and add a table. The ideal wrap up table contains three rows and single column. You can not set margins there but you can set the width of the table. In this sample document I used a table width of 90%.
Choose to align center in the Layout section if you wish your table to appear in the center with 5% margins at both sides.
After that we add our content in middle cell of the table.
Another layout could be adding a single row and single column table at the top, then a three column and single row table in the middle. In the three column table change cells one by one to set their width. More easier would be to go into HTML mode and change 33% to 5%,90% and 5%. (See sample document 2).
