I'm answering my own question because I found a way to achieve what I needed, but I'm not using persistentFooterButtons anymore, so I think there are more correct answers for this specific thread.
The property bottomNavigationBar doesn't have a hard coded padding, contrary to persistentFooterButtons (as noted by MendelG in the other answer).
Therefore, I can change the background color using a Container, and add the padding myself (inside the container) to obtain the wanted result:
Scaffold( // ... bottomNavigationBar: Container( color: Colors.white, child: Padding( padding: const EdgeInsets.all(8.0), child: Row( children: [ // ... ], ), ), ),),Result
