Why parent class is accepting color from child?

Question

I'm working on Slider, I want to change the thumb color, but this is not working. thumbColor accepts color from Child:Slider activeColor.

SliderTheme(

              data:SliderTheme.of(context).copyWith(
                // thumbShape: RoundSliderThumbShape(enabledThumbRadius: 19.0),
                // overlayColor: Colors.grey,
                thumbColor: Colors.white,

              ),
              child: Slider(
                inactiveColor: Colors.transparent,
                min: 0,

                activeColor: Color(0xffF3C961),
                value: value.toDouble(),
                max: MediaQuery.of(context).size.height,
                onChanged: (changedValue){
                  setState(() {

                    value = changedValue.toInt();

                  });
                },
              ),
            ),[![You can check color of thumb shape][1]][1]