In the 2.9 theme, the code is in this directory:

/include/classes/css.php

I would think it would be the same for 3.0 and above. Find the code that starts on line 22 in a code editor. It should look like this:

Code:
 public static function validate($css)
    {
        $valid_css = array(
            'default', 'black'       );
Change the 'default', 'black' to:

Code:
public static function validate($css)
    {
        $valid_css = array(
            'black', 'black'       );
Both have to be set to black for you to use the black theme exclusively. If you don't change it to that, it'll keep reverting to the default, which is the white theme.