DarkMediaWiki is a docker image coming from MediaWiki that includes a DarkVector theme from https://github.com/dolfinus/DarkVector.
To select the dark theme, add the following to the LocalSettings.php file:
wfLoadSkin( 'DarkVector' );
$wgDarkVectorUseSimpleSearch = true;
$wgDarkVectorUseIconWatch = true;
The skin must be set for each user. You can also set it as the default skin:
$wgDefaultSkin = 'DarkVector';
Also refer to the mediawiki docs here and DarkVector docs here.