Create subtheme manually

Copy the provus_bootstrap folder on your /themes/custom directory.

Rename the provus_bootstrap folder to your custom theme name, for example yourname

Rename the following files from provus_bootstrap to yourname: provus_bootstrap.theme to yourname.theme provus_bootstrap.info.yml to yourname.info.yml provus_bootstrap.libraries.ymlto yourname.libraries.yml /config/install/provus_bootstrap.settings.ymlto /config/install/yourname.settings.yml /config/schema/provus_bootstrap.schema.ymlto /config/schema/yourname.schema.yml

Edit the file yourname.info.yml

name: YourName
type: theme
description: 'Your Description.'
version: VERSION
core: 8.x
base theme: bootstrap_barrio
libraries:
- yourname/global-styling

Make sure that yourname/bootstrap_cdn is the Bootstrap library you want (CDN or local).

Delete the following:

# Text here
version: '8.x-4.17'
core: '8.x'
project: 'bootstrap_barrio'
datestamp: 1533828192

Edit /config/schema/yourname.schema.yml

# Schema for the configuration files of the Bootstrap Barrio Subtheme.

yourname.settings:
type: theme_settings
label: 'Yourname settings'

Edit yourname.theme

Rename the function from:

function provus_bootstrap_form_system_theme_settings_alter(&$form, FormStateInterface $form_state)

to:

function yourname_form_system_theme_settings_alter(&$form, FormStateInterface $form_state)

For a complete explanation on how to write sub-themes on Drupal 8 please refer to the Creating a Drupal 8 sub-theme Guide