url.css

html {
    font: normal 20px/1.5 sans-serif;
}

body{
    margin: 0;
    background: linear-gradient(60deg, rgba(84,58,183,1) 0%, rgba(0,172,193,1) 100%);
}

h1 {
    text-align: center;
    color: white;
}

h3{
    font-size: larger;
    color: white;
}

h4 {
    font-size: large;
    text-align: center;
    width: 340px;
}

a{
    color: rgb(56, 56, 236);
}

#longurl{
    width: 340px;
    overflow-wrap: break-word;
}

.header {
    position:relative;
    text-align:center;
    color:white;
    height: 100vh;
}

.flex { /*Flexbox for containers*/
    display: flex;
    justify-content: center;
    align-items: center;
    flex-direction: column;
    text-align: center;
  }

input{
    width: 300px;
    height: 40px;
    background: none;
    border: none;
    border-bottom: 2px solid white;
    margin: 15px;
    outline: none;
    font-size: 1.2rem;
    color: white;
    padding: 0 10px;
}

form, div {
    padding: auto;
    margin-top: 30px;
}

.form-box {
    clear: both;
    width: 90%;
    position: relative;
}

.form-box input {
    font-size: 1rem;
    width: 500px;
    margin-left: 50px;
    padding: 0.25rem 1rem;
    background: none;
}

.error::after {
    font-size: 1rem;
    line-height: 1.8;
    width: 350px;
    padding-left: 1rem;
    position: absolute;
    right: 0;
    content: attr(data-errormsg);
}

img{
    display: block;
    margin: 30px auto;
}

@media (max-width: 768px) {
    .form-box input{
        width: 230px;
    }
    .header {
      height:100vh;
    }
    h1 {
      font-size:24px;
    }
  }