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.

If you'd like to change your legend's border color to make it more noticeable, click the Format Legend's "Border" button and then click "Color" to display a list of colors.

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.