Manual:$wgStyleDirectory

Server URLs and file paths: $wgStyleDirectory
The file system path to the skins directory.
Introduced in version:1.3.0
Removed in version:still in use
Allowed values:(relative path)
Default value:null (1.38+)

"{$IP }/skins" (1.4-1.37)

"{$IP }/style" (1.3)

Details edit

The file system path to the skins directory. This controls where MediaWiki will look for skin files on the server itself.

Set to "{$IP}/skins" by Setup.php before loading local settings.
Ignored when the exact path to a particular skin's .json configuration file is given to the wfLoadSkin() function.
Example: wfLoadSkin( 'Vector2', '/path/to/Vector2/skin.json' );

See also edit