Cloning one of the mediawiki/skins/* repositories via git into the skins/ directory of your MediaWiki installation.
Doing this should not interfere with your git repository if you're a MediaWiki developer.
If you have just upgraded MediaWiki
MediaWiki 1.24 and newer no longer automatically enables installed skins (see Manual: Skin autodiscovery). You can paste the following lines into LocalSettings.php to enable all currently installed skins: