use this code

body {
    margin: 0;
    padding: 0;
}
.welcome-section {
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  width: 100%;
  height: 100vh;
  background-color: #000;
  margin: 0px;
  color: #fff;
}

<section id="welcome-section" class="welcome-section">
    <h1>Hello My name is Ben</h1>
    <p>A Web Developer </p>
  </section>

add in your css :-

body {
    margin: 0;
    padding: 0;
}

You could try adding border: 0 to your style rules. It sounds like by ‘margin’ you mean border

It’s likely something else on the page is setting a border. You really should share a full example though.

Try this,
Add this to your css

body{
    margin: 0%;
}

It will fit background color to the border.

For some reason the code below will output a black background that has white margins around it. I even have margin: 0px but for some reason the margins are still there. I have tried to set all the other elements to 0 margin but that didn’t work either.

.welcome-section {
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  width: 100%;
  height: 100vh;
  background-color: #000;
  margin: 0px;
  color: #fff;
}

<section id="welcome-section" class="welcome-section">
  <h1>Hello My name is Ben</h1>
  <p>A Web Developer </p>
</section>

.welcome-section {
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  width: 100%;
  height: 100vh;
  background-color: #000;
  margin: 0px;
}