I installed Bootstrap 3 for Drupal as the theme for my blog, but I want to change and add some minor things to that theme. For that purpose Drupal 8 offers the sub-theming mechanism. A sub-theme is a theme based upon another theme. Because I will base my sub-theme upon Bootstrap, I will call my sub-theme "My Bootstrap".
My Bootstrap sub-theme
In the directory "themes", in which I had already installed the Bootstrap theme, I create a new subdirectory "themes\mybootstrap". In that subdirectory I create a new empty file "mybootstrap.theme" and a file "mybootstrap.info.yml" with the following content:
name: 'My Bootstrap'
description: 'My adaptation of the Bootstrap 3 theme.'
base theme: bootstrap
navigation_collapsible: 'Navigation (Collapsible)'
header: 'Top Bar'
page_top: 'Page top'
page_bottom: 'Page bottom'
The line "base theme: bootstrap" tells Drupal that this theme is a sub-theme of bootstrap. The regions contain exactly the same regions as the Bootstrap theme on which it is based. This sub-theme is now an exact copy of Bootstrap.
The first thing I want to change is to add some CSS files to the sub-theme. For this I downloaded and saved the fontawesome toolkit to sub-theme subdirectory and in the new subdirectory "themes\mybootstrap\css" I placed a new empty file "mybootstrap.css" for later use.
To make fontawesome and my (still empty) css file available to the sub-theme I add the following to the "mybootstrap.info.yml" file:
The locations of these libraries are defined in the file "mybootstrap.libraries.yml" in the subdirectory "themes\mybootstrap":
We are going to use these css files to facilitate some of the changes in the sub-theme.