Refined Toolkit for Confluence is a Confluence app which includes a bundle of beautiful UI elements.

Announcing Refined Toolkit for Confluence Data Center

Data Center approved apps follow the same annual subscription model as your Data Center host products, meaning it will be a paid version of the app (you can read more about it here: 

Note that the Server version of Refined Toolkit will continue to be free of charge!

With Data Center approved app comes guarantees that the app has been tested to perform at scale and being eligible for our enhanced Data Center support SLA. It will also allow us to further improve the app and continue to add new features.
More information will be available when the release gets closer. If you have any questions regarding this, don't hesitate to contact us through our support.

Some additional information about Atlassian's Data Center Approval program can be found in the pdf below: