Drupal 9 Create Bootstrap 3 Subtheme

Follow below steps to create your own theme from bootsrap theme. Below steps are tested for Drupal 9 and Bootstrap 3.

Step 1: Download Bootstrap theme from or use below command to download

drush dl bootstrap

Step 2: Naviagte to bootstrap/starterkits folder and copy THEMENAME folder and paste it to main theme folder

Step 3: Rename THEMENAME to your theme name. I am renaming it to my_custom_theme

Step 4:  Rename below 3 files. Replace THEMENAME with your theme name and starterkit with info

THEMENAME.libraries.yml -> my_custom_theme.libraries.yml
THEMENAME.starterkit.yml -> my_custom_theme.info.yml
THEMENAME.theme -> my_custom_theme.theme

Step 5: Open config folder and rename THEMENAME.settings.yml file inside install folder

THEMENAME.settings.yml -> my_custom_theme.yml

Step 6: Open config folder and rename THEMENAME.schema.yml file inside schema folder

THEMENAME.settings.yml -> my_custom_theme.yml

Step 7: Open my_custom_theme.info.yml and change value for THEMETITLE and THEMENAME.  

type: theme
base theme: bootstrap
core_version_requirement: ^9 || ^10

name: 'THEMETITLE'
description: 'A Drupal Bootstrap 3 based sub-theme.'
package: 'Bootstrap'

regions:
  navigation: 'Navigation'
  navigation_collapsible: 'Navigation (Collapsible)'
  header: 'Top Bar'
  highlighted: 'Highlighted'
  help: 'Help'
  content: 'Content'
  sidebar_first: 'Primary'
  sidebar_second: 'Secondary'
  footer: 'Footer'
  page_top: 'Page top'
  page_bottom: 'Page bottom'

libraries-extend:
  bootstrap/framework:
    - THEMENAME/framework

Step 8: Click on Appearence and Install Theme