In a word, in Magento there are three maximum fallback steps implied by theme hierarchy when developing:
- Custom theme is a current website theme that is being developed.
- Default theme is a parent version from which we inherit Custom theme. It can be not only the preselected theme but also the theme that has been already designed.
- Base theme is the main Magento theme to be inherited from and reassigned by other themes (Custom and Default). One can say it is the part of Magento’s core. It is preferable not to alter files directly in the theme, but create a duplicate in the theme, as when updating Magento, all the changes in the folder “Basic theme” can be rerecorded.