iMac display

My configuration:
Model Name: iMac
Model Identifier: iMac8,1
Processor Name: Intel Core 2 Duo
Processor Speed: 3.06 GHz
Number of Processors: 1
Total Number of Cores: 2
L2 Cache: 6 MB
Memory: 4 GB
Display: 24 inch

My display does not show grid lines (and other horizontal lines) as found in Excel and Numbers. Any suggestions?

Make sure the grid lines are turned on.
Show or hide gridlines on a sheet - mac excel - Office.com

If they are and you still cannot see them, you may need to adjust your display brightness/contrast or calibrate your color profile.
How to Calibrate your Mac's Display | Mactuts+