How can I customize Slider widget in Flutter?

Wrap your slider with a SliderTheme

SliderTheme(
    data: SliderThemeData(
            thumbColor: Colors.green,
            thumbShape: RoundSliderThumbShape(enabledThumbRadius: 20)),
    child: Slider(
          value: _value,
          onChanged: (val) {
            _value = val;
            setState(() {});
          },
        ),
      ),