To do Alt + H B H on Mac in Excel, download and install Accelerator Keys, then press Option, followed by H, B, H.
Command: Thick Bottom Border
This is how to do Alt + H B H 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 H thick Bottom Border applies a thick border to the bottom of selected cells. This is useful for separating sections or emphasizing totals.