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.

To add a Border to one or more Cells in Microsoft Excel, open the spreadsheet, select the Cells, click on the Borders button and take it from there.

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.