'Flutter || Padding for floatingActionButton in flutter

In my flutter project I used a floatingActionButton and I want to add a padding but I don't know how to make the tree with my code. Any help is highly appreciated.

This is my code :

 floatingActionButton: Visibility(
              visible: buttonshow,
              child: FloatingActionButton(
                onPressed: () {
                  scrollToTop();
                },
                shape: const StadiumBorder(
                    side: BorderSide(
                        color: Color(0xff135888), width: 2.5)),
                backgroundColor: Colors.transparent,
                elevation: 0.0,
                child: const Icon(
                  Icons.keyboard_arrow_up, color: Color(0xff135888),
                  size: 28.0,),
                mini: true,

              ),
            ),


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