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: https://www.atlassian.com/blog/enterprise/data-center-approved-apps).
Note that the Server version of Refined Toolkit will continue to be free of charge!
Some additional information about Atlassian's Data Center Approval program can be found in the pdf below: