To change Gridline color in Excel, use Excel Options, apply a custom-colored border to the cells, or apply a custom line color. Full details here.

Select "Outside Borders" to select a 0.25 point line around the selected cells, or "Thick Box Border" to select a heavier stroke. You can change the "line styles" of the border.

The default active cell format in Microsoft Excel isn't all that easy to see, but you can use this easy-to-implement code solution to change that.