To do any customization on themes in big commerce, initially the theme needs to be downloaded. Downloading a theme from the control panel allows to have a copy on computer. A copy is needed if choose to develop a custom theme using the Stencil CLI. Making changes to your theme in CLI gives you full access to JavaScript files that are not available when using the theme editor in the control panel.
To download theme file, Storefront -> Themes -> Advanced -> Make a copy.
In the copied theme, in action menu-> Customize -> Edit theme files

Adding Custom Scripts to Stencil Themes:
- USING THE SCRIPT MANAGER
To access the Script Manager, go to Storefront › Script Manager.
To add a script, click Create a Script.
Enter the details such as name of script, description, location on page, Pages where script will be added, script category, script type and load method. Click on save.
