Apple iWork '08 - Manual (Page 79)

Chapter 2 Getting Started with Numbers 79
To format a border at the bottom of the table:
1 Click the reference tab for the footer row to select all the cells in the row.
2 In the Format Bar, choose Bottom Edge from the Border Selection pop-up menu.
Bottom Edge indicates you want to format a border for the bottom segment of each
selected cell.
3 Choose a line style and a stroke thickness for the border from the two adjacent pop-up
menus.
4 Click the color well to choose a color for the border.
Select Bottom Edge to add a
border to the bottom edge of
selected cells.
Change the stroke thickness.
Choose a line (stroke) style.
Choose a stroke color.