How to remove white space behind the picture

How can I make that background around the picture transparent.

Here is the code of the screen:

body: Column(
          children: [
            Expanded(child: SingleChildScrollView(child: Text("loooong text"))),
            TypeChallengeWidget(),
          ],
        )

And this is the TypeChallengeWidget:

Widget build(BuildContext context) {
    final screenHeight = MediaQuery.of(context).size.height;
    final screenWidth = MediaQuery.of(context).size.width;
    return Stack(
      children: [
        Container(
          margin: EdgeInsets.only(top: screenHeight * 0.1 / 2),
          decoration: BoxDecoration(
            color: Colors.transparent.withOpacity(0.5),
            borderRadius: BorderRadius.only(topLeft: Radius.circular(15), topRight: Radius.circular(15))),
          child: Padding(
            padding: EdgeInsets.symmetric(vertical: screenHeight * 0.1/1.5, horizontal: 20),
            child: Column(
              crossAxisAlignment: CrossAxisAlignment.start,
              children: [
                Text("Test:"),
                Row(children: [
                  const Flexible(child: TextField(keyboardType: TextInputType.name, autocorrect: false,)),
                  ElevatedButton(
                    style: ElevatedButton.styleFrom(
                      backgroundColor: Colors.green,
                      shape: const CircleBorder(),
                      padding: const EdgeInsets.all(20),
                    ),
                    onPressed: (){}, 
                    child: const Text(">"))
                ],)
                
              ],
            ),
          ),
        ),
        Align(
          alignment: Alignment.topCenter,
          child: SvgPicture.asset("img", height: screenHeight*0.1,)),
      ]
      );
  }

I tried to put Stack into container and make it transparent but it doesn't work.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-html -->

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    final screenHeight = MediaQuery.of(context).size.height;
    final screenWidth = MediaQuery.of(context).size.width;
    return Column(
      children: [
        Expanded(
          child: SingleChildScrollView(
            child: Wrap(
              children: List.generate(300, (index) => Text("loooong text")),
            ),
          ),
        ),
        Stack(
          alignment: Alignment.topCenter,
          fit: StackFit.passthrough,
          clipBehavior: Clip.none,
          children: [
            Container(
              // margin: EdgeInsets.only(top: screenHeight * 0.1 / 2),
              decoration: BoxDecoration(
                color: Colors.transparent.withOpacity(0.5),
                borderRadius: const BorderRadius.only(
                  topLeft: Radius.circular(15),
                  topRight: Radius.circular(15),
                ),
              ),
              child: Padding(
                padding: EdgeInsets.symmetric(
                    vertical: screenHeight * 0.1 / 1.5, horizontal: 20),
                child: Column(
                  crossAxisAlignment: CrossAxisAlignment.start,
                  children: [
                    const Text("Test:"),
                    Row(
                      children: [
                        const Flexible(
                            child: TextField(
                          keyboardType: TextInputType.name,
                          autocorrect: false,
                        )),
                        ElevatedButton(
                            style: ElevatedButton.styleFrom(
                              backgroundColor: Colors.green,
                              shape: const CircleBorder(),
                              padding: const EdgeInsets.all(20),
                            ),
                            onPressed: () {},
                            child: const Text(">"))
                      ],
                    )
                  ],
                ),
              ),
            ),
            Positioned(
              top: -(screenHeight * 0.1 / 2),
              child: SvgPicture.asset(
                "assets/report.svg",
                height: screenHeight * 0.1,
              ),
            ),
          ],
        ),
      ],
    );
  }
}

<!-- end snippet -->

You can add 2 params to the stack widget
alignment: Alignment.topCenter,
clipBehavior: Clip.none,

and refactor the image widget with positioned and remove the margin from container to the mince top positioned widget

hope this will help