How can I add new page on GitHub page sharing the same theme with index.md?

Currently, I have one main page on my GitHub account (https://<username>.github.io/). I would like to add another page that shares the same theme with the main page.(https://<username>.github.io/<new_page>)

I have tried adding new markdown file (e.g. <new_page>.md) with this:

---
layout: page
title: "<new_page>"
permalink: /<new_page>/
---

at the top of file, but it didn't work. Also, tried to add theme: <same_theme> but it didn't work as well.

Note that the theme that I used is jekyll-theme-minimal.

Below is the example of my page:

I would like to add another page with the same header on the left-hand side pane, just vary the right-hand side content.