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:
https://marketplace.atlassian.com/search?query=Brikit%20Theme%20Press

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:
https://developer.atlassian.com/confdev/tutorials/writing-a-confluence-theme?_ga=1.265485647.1517411526.1490638576

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 .atlassian.net it’s Cloud)

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

Thanks for helping me!!