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.

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.

How to Make Colors Change With Validation in Excel. Excel's conditional formatting tool applies formatting depending on cells' contents. Validation rules can change cells font, style, size or border.