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.

In case of an error, a triangle appears in the corner of the cell. You can change the Color of an Error Indicator from green to any other in Excel.

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.