Installation

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drush -y en bootstrap

Now you should see the theme in /themes:

Copy the CDN folder from the themes/bootstrap/starterkits folder..

..and paste it in the /themes folder:

Change the cdn folder name to mytheme and all THEMENAME occurrences to mytheme:

From To
mytheme/config/install/THEMENAME.settings.yml mytheme/config/install/mytheme.settings.yml
mytheme/config/schema/THEMENAME.schema.yml mytheme/config/schema/mytheme.schema.yml
mytheme/THEMENAME.starterkit.yml mytheme/mytheme.info.yml
mytheme/THEMENAME.libraries.yml mytheme/mytheme.libraries.yml
mytheme/THEMENAME.theme mytheme/mytheme.theme

Note: change .starterkit.yml to .info.yml

Open mytheme/mytheme.info.yml

Change description to your liking

Change all THEMENAME occurences to mytheme:

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core: 8.x
type: theme
base theme: bootstrap

name: 'My Theme'
description: 'Uses the jsDelivr CDN for all CSS and JavaScript. No source files or compiling is necessary and is recommended for simple sites or beginners.'
package: 'Bootstrap'

...

libraries:
  - 'mytheme/global-styling'

Open mytheme/config/schema/mytheme.schema.yml

Change all THEMENAME occurences:

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mytheme.settings:
  type: theme_settings
  label: 'Mytheme settings'

Go to admin/appearance and FIRST enable the bootstrap basetheme (read more).

After that enable the custom theme:

And there you have it:

Bootswatch

Go to admin/appearance/settings/mytheme to change the theme settings. For example, in the Advanced section you can select Bootswatch themes:

The Paper theme looks like this:

Cerulean theme:

CSS Styles

Disable caching (check this tutorial on how to do it).

Override the default styling in themes/mytheme/css/style.css:

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h1 { color: red;}

Now you should see red main headers: