To do Alt + H B S on Mac in Excel, download and install Accelerator Keys, then press Option, followed by H, B, S.
Command: Outside Borders
This is how to do Alt + H B S 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 S outside Borders applies borders around the outside edge of the selected range. The interior cell boundaries remain without borders.