I have not seen any documentation on this, so I thought I should post information about the built-in CSS version query string in Sitefinity.
By default, when you add a css file to your theme, Sitefinity will add a query string with the version number such as style.css?v=60743210. This is very helpful for making sure that when you push updates your users immediately get your updated styles.
There is one important thing to know in order to get this to work. As far as I can tell, your theme must be in the ~/App_Data/Sitefinity/WebsiteTemplates/[YourFolder]/App_Themes/[YourTheme]/ folder, otherwise the query string will not be added. It does not work if your theme is in ~/App_Themes/[YourTheme]/