|Site customization: $wgStyleVersion|
|Appended in the query string of static CSS and JS to clear client side cache when changing this.|
|Introduced in version:||1.9.0 (r16992)|
|Removed in version:||still in use|
|Default value:||(not fix)|
|Other settings: Alphabetical | By function|
It should be appended in the query string of static CSS and JS includes, to ensure that client-side caches don't keep obsolete copies of global styles.
You don't need to change this. In addition the default values are not documented, as the actual value of this setting has no significance (except to note that it will be different, and higher, in each version of MediaWiki).