Wikimedia Bugzilla is closed!

We see

/**
 * Make the logos configurable.
 *
 * 'logo' is the principle logo for your site, 'copyright' is the copyright
 * logo to be used in the footer of your site.
 *
 * Example: array(
 * 	'logo' => 'mysite_logo.png',
 *	'copyright' => 'mysite_copyright_logo.png',
 * 	);
 */
$wgMFCustomLogos = array();

However having the user set $wgMFCustomLogos['logo'] does nothing, as
it is nowhere else in the code! P.S., principal is spelled wrong. P.S.S.
if one day it is actually used, please mention where one can expect to
see it appear.