body.login { 
    background-color: #000; 
    /* or */
    background-image: url('path-to-your-background.jpg');
    background-size: cover;
}
.login form {
    background: #ffffff;
    border-radius: 8px;
    box-shadow: 0 4px 6px rgba(0,0,0,0.1);
}
.wp-core-ui .button-primary {
    background-color: #0073aa;
    border-color: #0073aa;
}