Create content formatting for confluence on private network

I’m a JAVA Developer, but new to Confluence. We have Atlassian Confluence 8.7.1 on a private network, and the customer would like me to develop a app that will allow users to format their content. This would be similar to the “Content Formatting for Confluence” app, which provides access to a range of macros that let users add rich Atlassian Design Guidelines compliant elements to the Confluence pages.

Is this possible, the customer would like it custom developed to specific requirements since it it on a private secure network that is behind a firewall and does not have access to the internet.

Please advise. Thank you