Keyboard shortcut for buttons in dialogue box
Almost, though not quite, universal on macOS is the use of  Cmd ⌘   Backspace ⌫  [or Delete on keyboards without a distinct Backspace key] to discard, or discard changes.

This should also work to invoke "Don't Save" in dialogs too.
Some apps still accept the alternative of  Cmd ⌘   D  to do this, but Backspace has been the recommended method for several years [since Lion or so].