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