Modifying wp-login.php

in the CSS file,
body {
background: #fff;
color: #000;
margin: 0;
padding: 0;
}
would change it to white… but note that this will change the background throughout the admin interface.

Sure, create a seperate CSS file just for the login page (then edit the login page so it looks at the special CSS file) and then put the code below in that CSS file:
body {
background: #000;
color: #000;
margin: 0;
padding: 0;
}