If you don't already have pixie you can download it (for free) here:
http://www.nattyware.com/pixie.php
With this program you can find out that Excel's normal grid lines have RGB code (212, 212, 212).
Now select B11:G16, press Format, Format Cells, Color, More Colors, Custom, enter 212 in each of the fields Red, Green and Blue, press OK, press Outline and press OK.