index.css

body{
    background-color: #222;
    font-family: Georgia;
    box-sizing: border-box;
}

input{
    display:block;
    width:100%;
    padding:10px;
    margin: 10px auto;
    border: none;
    box-sizing: border-box;
    background:transparent;
    border-top: 2px solid #555;
    outline: none;
    transition: color 0.3s, background 0.3s;
    color:#bbb;
    

}

input::placeholder{
    color: #bbb;
    font-weight:800;
}

input:hover {
    background:#bbb;
    
}

input:hover::placeholder{
    color:#333;
}

input[type="submit"]{
    width: 50%;
    margin:auto;
    background: #bbb;
    font-weight: 600;
    color: #333;
}

input[type="submit"]:hover{
    background:#777;
}

input[type="datetime-local"]{
    background: #bbb;
    font-size: large;

}

input:focus{
    background: #bbb;
    color: #000;
    
}

form{
    width:80%;
    margin:auto;
    padding: 30px;
}

p{
    color: #bbb;
    font-weight:800;
    padding:5px;
    text-align: center;

}

h2{
    color:#bbb;
    text-align: center;
}

.resultados{
    margin:2%;
    font-size: x-large;
}
.green{
    color:#5f5;
}

.red{
    color: #f22;
}
span {
    display:none;
}

input + span {
    padding-right: 30px;
}
  
input:invalid+span:after {
    position: absolute;
    content: '✖';
    color:#f22;
}
  
input:valid+span:after {
    position: absolute;
    content: '✓';
    color:#5f5;
}