Themes
Jekyll themes provide community-maintained templates and slides for users to build their websites. While the basic Minima theme is very robust, you can choose themes from many free resources, such as:
You can clone or fork the themes available on Github. For other themes, you need to download and unzip them on your local machine. Then, serve it using Jekyll. Remember that you need to install the gems used by third-party themes before serving them by running bundle exec jekyll serve the first time you serve them on your local machine.
HTML
Hyper Text Markup Language (HTML) was created to describe the content of a web page. It is a markup language that consists of a series of elements. The elements tell the browser how to display the content. An HTML element is defined by a start tag, some content, and an end tag. The web browser reads HTML files and uses them to display the content on your screen.
You can build a website by writing an HTML file from the scratch. However, such a task is error-prone and time-consuming. Moreover, if any changes need to be made across all pages, you need to change the HTML files one by one. Website generators are created to simplify this process.
CSS
Jekyll provides built-in support for Sass, a CSS extension language. Cascading Style Sheets (CSS) defines styles for your web pages by describing how HTML elements are to be displayed on different screens. By setting properties in a single file, CSS files enable users to change the look of an element over the entire website by changing just one line.
Most of the themes put their styling as either direct CSS or partial Sass files in a _assets directory. If you are interested to change the font, colour, or other properties of HTML elements to customize the webpage, you need to find that element in the styling files and change its properties accordingly.
Gem-based Themes
In a recent update, Jekyll introduced gem-based themes, which stores some of the site’s directories, such as assets, _layouts, _includes, and _sass directories) in the theme’s gem. If you used jekyll new to build your first website, you are using Minima and cannot see the abovementioned folders. While gem-based themes make it easier to package a theme and are more robust, you need to manually find those files and copy them into your website directory to override theme default.
For more information, take a look at the Jekyll post on Overriding theme defaults
If you are interested to use themes other than Minima, Hyde is a simple, two-column, and uncomplicated theme. However, it does not compile without error with the current version of Jekyll. If you are ready for some troubleshooting, give it a shot. If you like to start with simpler options, check out Jekyll Now.