estilo.css

body {
    background-image: url(backgroundtest.jpg);
    display: flex;
    justify-content: space-around;
    align-items: center;
    min-height: 100vh;
    color: #fff;
}

.container {
    min-width: 400px;
    min-height: 400px;
    background-color: #1d1e33;
    padding: 2rem;
}

#f-cont h1 {
    text-align: center;
    margin-bottom: 2em;
}

.f-input {
    display: flex;
    flex-direction: column;
}

.form-green {
    border-color: green;
}

.form-red {
    border-color: red;
}

.form-control {
    display: flex;
    flex-direction: column;
    margin-bottom: 1rem;
}

button {

    justify-items: center;
    text-transform: uppercase;
    border: black;
    color: azure;
    cursor: pointer;
    background-color: cornflowerblue;
    padding: 1rem 1.5rem;
    flex: 1;
    opacity: 0.8;
}

button:hover {
    opacity: 1.2;
}