QR Code Component using HTML & CSS

I see you have set height of the body to be 90vh which is the reason the box isn't centered exactly but moved a little towards the top.

You can fix this by setting padding and margin to be 0px globally and the box-sizing to border-box and then using 100vh instead of 90 as follows:

* {
      box-sizing: border-box;
      padding: 0px;
      margin: 0px;
      --slate-900: #1f314f;
      --slate-500: #68778d;
      --slate-300: #d5E1EF;
      --white: #fff;
      --card-container-gap: 24px;
      --card-container-padding: 16px;
      --card-container-padding-bottom: 40px;
    }

.container{
     height: 100vh
}