Stephanie,
In Excel 2007 click the 'Office Button' then click the [Excel Options]
button near the lower right corner of the displayed information.
In the Options window choose [Advanced], scroll down some and you'll find a
section on "Display Options for this Worksheet" and it has the option to
hide/display gridlines and to change their color!
It's a per-sheet setting (not my fault!). I even tried selecting all sheets
in the workbook and then changing this setting, but at that point it showed
"Multiple Selected Worksheets" as the chosen sheet and none of them got
changed.