<button disabled>Enviar</button>

button {
    background-color: blue;
    color: white;
    border-radius: 50px;
    padding: 1em 2em;
    font-size: 1rem;
    outline: 0;
}

button:disabled {
    opacity: 0.5;
    cursor: not-allowed;
}

/* No aplicable en button disabled */
button:focus {
    box-shadow: 0 0 0 2px white, 0 0 0 4px blue;
}

/* No aplicable en button disabled */
button:active {
    background-color: DarkBlue;
    box-shadow: 0 0 0 2px white, 0 0 0 4px DarkBlue;
}