To do Alt + H B B on Mac in Excel, download and install Accelerator Keys, then press Option, followed by H, B, B.
Command: Bottom Double Border
This is how to do Alt + H B B on Mac in Excel with Accelerator Keys, the native ribbon and alt key shortcut solution for Mac users who get more done with Excel.
Alt + H B B bottom Double Border applies a double-line border to the bottom of selected cells. This style is commonly used in accounting to indicate grand totals.