How to remove the border or padding from CustomDropdown in Flutter using Dart?

enter image description here

I'm using the animated_custom_dropdown library version 3.1.1.

I've run into this problem:

By default, CustomDropdown comes with a transparent padding or border on the left and right sides.

Without adding any decoration:

  CustomDropdown<String>(
                    hintText: 'Seleccionar categoría',
                    items:
                        _categoryItems
                            .map((e) => e.description ?? 'Sin nombre')
                            .toList(),
                    initialItem:
                        _categoryItems
                            .firstWhere(
                              (e) => e.id == _selectCategory,
                              orElse: () => _categoryItems.first,
                            )
                            .description ??
                        'Sin nombre',
                    onChanged: (value) {
                      final selected = _categoryItems.firstWhere(
                        (e) => e.description == value,
                      );
                      setState(() => _selectCategory = selected.id);
                      debugPrint('cambiando valor a: $value');
                    },
                  ),

It is shown like this

enter image description here

And if I add a decoration: with the

                      closedFillColor: Colors.transparent,

decoration: CustomDropdownDecoration(
                      expandedBorder: Border.all(
                        color: Theme.of(context).primaryColor,
                      ),
                      closedFillColor: Colors.transparent,
                      expandedFillColor: Colors.white,
                      closedSuffixIcon: const Icon(Icons.keyboard_arrow_down),
                      expandedSuffixIcon: const Icon(Icons.keyboard_arrow_up),
                    ),

It looks like this:

Enter image description here

But what if I add a

                      closedBorder: Border.all(color: Colors.grey.shade400),

and I changed closedFillColor from transparent to white, so now it looks like this:

enter image description here

I'm not very familiar with the library. What I want is to remove that border or padding so that the Expanded field occupies the entire space. Thank you very much.