lightdm-mobile-greeter/style.css

window * {
    background: none;
    color: greenyellow;
    box-shadow: none;
    text-decoration: none;
}

window {
    background-color: black;
}

button {
    border: 0;
    background: none;
    outline-style: dashed;
    margin: 0;
    border-width: 2px;
    padding: 16px 36px;
}

label {
    font-weight: bold;
}

button:active {
    background-color: greenyellow;
    color: black;
}

button:active label {
    color: black;
}

hdykeypad > button {
   font-size: 20px;
   font-weight: bold;
   padding: 10px;
}