To use this package, just use CustomUnderlineTabIndicator like this:

                 TabBar(
                      isScrollable: true,
                      labelColor: AppColors.textWhite,
                      unselectedLabelColor:
                          AppColors.textWhite.withOpacity(0.4),
                      indicatorColor: AppColors.textWhite,
                      indicatorWeight: 2.0,
                      indicator: const CustomUnderlineTabIndicator(
                        gradient: LinearGradient(
                          colors: [
                            Color(0xff292828),
                            Color(0xff373737),
                            Color(0xff404040),
                            Color(0xffAAAAAA),
                            Color(0xff404040),
                            Color(0xff373737),
                            Color(0xff292828),
                          ],
                        ),
                      ),
                      tabs: const [
                        Tab(
                          text: "Rewards",
                        ),
                        Tab(
                          text: "Offers",
                        ),
                      ],
                    ),