Apply Or Remove Cell Borders On A Worksheet - Microsoft Support

To apply a new or different border style, select the arrow next to Borders Button image, and then select a border style.

Tip: To apply a custom border style or a diagonal border, select​​​​​​​ More Borders. In the Format Cells dialog box, on the Border tab, under Line and Color, select the line style and color that you want. Under Presets and Border, select one or more buttons to indicate the border placement. Two diagonal border buttons Button image   Button image are available under Border.

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