I have tried to center my divs like described on this website but something is wrong and I don't get it. Maybe you know what the problem is, because I can't find a proper solution to center the divs in the exact middle. I have added just one css input description because the others are the same.

.registerBox {
    position: relative;
    height: 80%;
    justify-content: center;
    align-items: center;
  }
  .center {
    position: relative;
    height: auto;
    justify-content: center;
    align-items: center;
  }
  .centergui {
    justify-content: center;
    align-items: center;
  }
//text styles
.h1 {
  margin: auto;
  justify-self: center;
  text-align: center;
}
input[id="usernametext"]  {
  z-index: 2;
  border-radius: $textinputradius;
  background-color: $textinputbackground;
  font-size: $textinputfontsize;
  @include respond-to(medium-devices) {width: 125px;}
  @include respond-to(wide-screens) {float: none;}
}
  input[id="usernametext"],input[type="password"],input[id="emailtext"],input[id="phonenumbertext"], textarea, select {
      outline: #272727;
      border: none;
      color: #ececec;
    }<div className={"h1"}> <h1>Register</h1> </div>
        <div className={"flexCenter"}>
                <div className={"registerBox"}>
                    <div className={"center"}><input type={"text"} id={"usernametext"} placeholder={"Username"}/></div>
                    <div className={"center"}><input type={"text"} id={"emailtext"} placeholder={"Email-Address"}/></div>
                    <div className={"center"}><input type={"password"} id={"passwordtext"} placeholder={"Password"}/></div>
                    <div className={"center"}><input type={"text"} id={"phonenumbertext"} placeholder={"Phonenumber(optional)"}/></div>
                </div>
              
    