How to build a theme similar to "Brikit Theme Press for Confluence"

I need to build a theme for Confluence and it will be used for our intranet website. At Atlassian marketplace, I found that that

Brikit Theme Press for Confluence:

is close to what I need. By “close”, I mean its ability to customize, not its visual look and feel. I would like to learn/know how it was implemented.

I am a Java developer and new to Confluence/Jira. Where can I find a good tutorial explaining how to build a custom theme similar to “Brikit Theme Press for Confluence”? What I can find online such as the following is too simple:

Writing a Confluence Theme:

Any pointer is really appreciated.

Thanks a lot!!!

What version of Confluence is your instance you’re trying to theme? Cloud or Server? (Easy way to tell is if your url has it’s Cloud)

We are going to buy a license and install it on our own server.

Thanks for helping me!!