To add borders to a table, right-click anywhere on the table and choose Format (Table) or choose Format (Table) from the Edit menu. This will launch the Format (Table) dialog.

Borders outline the table on the printed page. Different borders are available for each side of the table.
The options are:
None – no border on this side.
Thin – a normal line border on this side.
Thick – a "double" line border on this side.
Using Format (Table) will set the borders for the current table. To set the borders for all tables in the file, use preferences.
See Also: Add Borders to Cells