If you created your project with Varbase Project, you could use the ready create-new-vartheme command. https://github.com/Vardot/varbase-project​
By Composer:
cd PROJECT_DIR_NAME/docroot/profiles/varbasecomposer create-new-vartheme "THEME_NAME" "ltr" "sites/default/themes/custom"
By Bash:
cd PROJECT_DIR_NAME/docroot/profiles/varbase/scriptsbash ./create-new-vartheme.sh "THEME_NAME" "ltr" "sites/default/themes/custom"
By Composer:
cd PROJECT_DIR_NAME/docroot/profiles/varbasecomposer create-new-vartheme "THEME_NAME" "rtl" "sites/default/themes/custom"
By Bash:
cd PROJECT_DIR_NAME/docroot/profiles/varbase/scriptsbash ./create-new-vartheme.sh "THEME_NAME" "rtl" "sites/default/themes/custom"
By Composer:
cd PROJECT_DIR_NAME/docroot/profiles/varbasecomposer create-new-vartheme "THEME_NAME" "ltr"
By Bash:
cd PROJECT_DIR_NAME/docroot/profiles/varbase/scriptsbash ./create-new-vartheme.sh "THEME_NAME" "ltr"
​https://github.com/Vardot/varbase/blob/8.x-4.x/scripts/create-new-vartheme.sh​
Copy the VARTHEME_SUBTHEME folder to your custom theme location.
Rename VARTHEME_SUBTHEME.starterkit.yml your_subtheme_name.info.yml
Rename VARTHEME_SUBTHEME.libraries.yml your_subtheme_name.libraries.yml
Rename VARTHEME_SUBTHEME.theme your_subtheme_name.theme
Rename VARTHEME_SUBTHEME.settings.yml
Rename VARTHEME_SUBTHEME.schema.yml
Rename VARTHEME_SUBTHEME optional blocks.
Rename VARTHEME_SUBTHEME.base.css files.
Rename VARTHEME_SUBTHEME-rtl.base.css files.
Rename VARTHEME_SUBTHEME.base.less file.
Rename VARTHEME_SUBTHEME-rtl.base.less file.
Replace all VARTHEME_SUBTHEME with the machine name of your theme.
Replace the name: 'Vartheme Sub-Theme (LESS)' to the name of your theme.
14 If we want to use the LTR (Left To Right) bootstrap.
15.1 Delete the template folder bootstrap.
15.2 Download the bootstrap library. change the version as you need.
15.3 Extract the bootstrap library.
15.4 Delete the archived bootstrap library.
16 If we want to use the RTL (right to left) bootstrap.
16.1 Delete the template folder bootstrap-rtl.
16.2. Download the bootstrap library. change the version as you need.
16.3. Extract the bootstrap library.
16.4. Delete the archived bootstrap library.
All command for steps are in this link​
​https://github.com/Vardot/varbase/blob/8.x-4.x/scripts/create-new-vartheme.sh​