Manual:$wgStyleSheetDirectory

Server URLs and file paths: $wgStyleSheetDirectory
The file system path of the directory containing the stylesheets.
Introduced in version:Before 1.1.0
Removed in version:1.5.0
Allowed values:(absolute path)
Default value:&$wgStyleDirectory (1.3-1.4)
"{$IP }/style" (pre 1.1 - 1.2)

Details

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The file system path of the directory containing the stylesheets. (Or the skins directory.)

This has been deprecated since v1.5.0. $wgStyleDirectory should be used instead.