Manual:$wgStyleVersion/en

Details
Bump this number when changing the global style sheets and JavaScript.

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).