How to customize Slider widget in Flutter?

I think you have to use the SliderTickMarkShape class

Base class for Slider tick mark shapes.

Create a subclass of this if you would like a custom slider tick mark shape.

The easy way is to get the actual SliderTheme used in your context and modify only the properties you need. For example, to modify one slide:

SliderTheme(
    data: SliderTheme.of(context).copyWith(
    activeTrackColor: Colors.white,
    thumbShape: RoundSliderThumbShape(enabledThumbRadius: 15.0),
    overlayShape: RoundSliderOverlayShape(overlayRadius: 30.0),
    ),
    child: Slider(
             value: height.toDouble(),
             min: 120.0,
             max: 220.0,
             activeColor: Colors.white,
             inactiveColor: Color(0xFF8D8E98),
             onChanged: (double newValue) {
                 setState(() {
                        height = newValue.round();
                      });
                    },
                  ),
                ),

Another option is modify the theme you're using in your app; in this way you modify all the sliders in the app:

MaterialApp(
      theme: ThemeData.dark().copyWith(
          sliderTheme: SliderTheme.of(context).copyWith( //slider modifications
            thumbColor: Color(0xFFEB1555),
            inactiveTrackColor: Color(0xFF8D8E98),
            activeTrackColor: Colors.white,
            overlayColor: Color(0x99EB1555),
            thumbShape: RoundSliderThumbShape(enabledThumbRadius: 15.0),
            overlayShape: RoundSliderOverlayShape(overlayRadius: 30.0),
          ),
          primaryColor: Color(0xFF0A0E21), // theme color
          scaffoldBackgroundColor: Color(0xFF0A0E21)), // theme background color
      home: InputPage(),
);

I remember , I had the same challenge.

I create my own wave slider:

import 'dart:math';

import 'package:flutter/material.dart';

List<int> bars = [];
const barWidth = 5.0;
double screenWidth;
int numberOfBars;

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Home(),
    );
  }
}

void randomNumberGenerator() {
  Random r = Random();
  for (var i = 0; i < numberOfBars; i++) {
    bars.add(r.nextInt(40) + 10);
  }
}

class Home extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    if (bars.isEmpty) {
      screenWidth = MediaQuery.of(context).size.width;
      numberOfBars = screenWidth ~/ barWidth;
      randomNumberGenerator();
    }
    return Container(child: WaveSlider());
  }
}

class WaveSlider extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => WaveSliderState();
}

class WaveSliderState extends State<WaveSlider> {
  double bar2Position = 180.0;

  _onTapDown(TapDownDetails details) {
    var x = details.globalPosition.dx;
    print("tap down " + x.toString());
    setState(() {
      bar2Position = x;
    });
  }

  @override
  Widget build(BuildContext context) {
    int barItem = 0;
    return Scaffold(
      body: Center(
        child: Stack(
          alignment: Alignment.centerLeft,
          children: <Widget>[
            GestureDetector(
              onTapDown: (TapDownDetails details) => _onTapDown(details),
              onHorizontalDragUpdate: (DragUpdateDetails details) {
                setState(() {
                  bar2Position = details.globalPosition.dx;
                });
              },
              child: Container(
                child: Row(
                  crossAxisAlignment: CrossAxisAlignment.end,
                  mainAxisAlignment: MainAxisAlignment.start,
                  children: bars.map((int height) {
                    Color color = barItem + 1 < bar2Position / barWidth
                        ? Colors.deepPurple
                        : Colors.blueGrey;
                    barItem++;
                    return Container(
                      color: color,
                      height: height.toDouble(),
                      width: 5.0,
                    );
                  }).toList(),
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

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(() {});
          },
        ),
      ),