You cannot make a button with backGround behavior, only a group. And even with a group, you would have to place it on all cards explicitly if you had not created that backGround group at the beginning. The automatic placement of a group on new cards has to be in place before those new cards are created.
From: Jim Hurley via use-livecode <[hidden email]>
To: use-livecode <[hidden email]>
Cc: Jim Hurley <[hidden email]>
Sent: Mon, Mar 27, 2017 12:25 pm
Subject: Retroactive background behavior
As I recall there is a method of retroactively creating a button with background behavior, this is appears on all existing cards.