More specifically, you must read and understand this section if:
You are customizing a site that implements the Kilimanjaro release of SCA or earlier.
If you are implementing the Aconcagua Release of SCA or later, the best practice is to use themes and extensions to customize your site. To customize HTML or Sass files for these implementations, you must use the theme developer tools. See Theme Developer Tools and Extension Developer Tools for details on using these tools.