To do Alt + H B I on Mac in Excel, download and install Accelerator Keys, then press Option, followed by H, B, I.
Command: Line Color
This is how to do Alt + H B I 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 I line Color lets you choose the color for borders you draw. The selected color applies to new borders created with Draw Border tools.