I personally had issues when trying to customize this Jekyll blogsite for the first time because Github’s own guide didn’t mention you should also change front matter “layout” variable for each page in your site to match what is included by the theme.

5) Add a new line to the file for the theme name.

To use a supported theme, type theme: THEME-NAME, replacing THEME-NAME with the name of the theme as shown in the _config.yml of the theme’s repository (most themes follow a jekyll-theme-NAME naming convention). For a list of supported themes, see Supported themes on the GitHub Pages site. For example, to select the Minimal theme, type theme: jekyll-theme-minimal.

To use any other Jekyll theme hosted on GitHub, type remote_theme: THEME-NAME, replacing THEME-NAME with the name of the theme as shown in the README of the theme’s repository.

Simply following the above step can break the styling of your page if your new theme doesn’t have the same set of layouts.

For example, if you start by using the minima theme which includes the layouts base, home, page, post, then change to the theme midnight which only has the layout default - all your pages will be displayed as an unstyled white-page. Instead, you must also change your page front matter to include layout: default like so:

---
layout: default # Assuming new theme is midnight
title: Your post title
date: 2000-01-01
# Any other variables
---

The following list includes the layouts included in the 13 Jekyll themes provided at github/pages-themes, as well as Jekyll’s own minima theme for reference.