There are number of improvements to the way themes are managed and set up in Magento 2. The use of the theme.xml definition file, introduced in Magento 1.9, and a new fallback system, are two of the most significant improvements. The fallback system in Magento 2 works in a similar way to Magento 1.x, but has the added advantage that you can select unlimited parent themes to inherit from / fallback to – all via the theme.xml file in your theme.
For the sake of compatibility, upgradability, and easy maintenance, do not modify the out of the box Magento 2 themes. To customize the design of your Magento 2 store, create a new custom theme. This article will show you how to create a Magento 2 theme step by step.