Individual cells have very few attributes that can be changed. Font, color, indentation, alignment, custom text, and number of decimal places are the only ones available.

Borders can only be created for cells by setting the borders on the rows and columns that contain the cell. To do this, right-click on the associated stub, caption or column head and choose Format (Stub), Format (Caption) or Format (Column Head) to bring up the Format (Table Part) dialog. For columns, set the left and right lines and for rows set the top and bottom lines. To ensure that line settings are applied to the entire row or column, make sure that Extend Lines into Cells is checked.

The column header "No" has thick lines set for left and right
and extend into cells is checked
You can also set the left and right borders of a spanner and have them extend into the leftmost and rightmost columns under the spanner. This is done the same way it is done for individual columns. Right-click on the spanner, choose Format (Spanner), set the left and right lines and ensure that Extend Lines into Cells is checked.

By default, stubs and columns have no borders, but spanners have thin lines on the left. This means that cells in the leftmost columns under a spanner will have left borders. You can change these default settings by using the Preferences.
See Also: Formatting Row, Column, or Cell Data, Add Borders to a Table