PrestaShop 1.7 introduces a totally reworked theme system from version 1.6, with the goal of making PrestaShop upgrades easier, and making it easier and faster to create a brand new theme.
The huge change to the theme system means that there is no easy way to migrate your PrestaShop 1.6 theme to PrestaShop 1.7.
When using the automatic upgrade from PrestaShop 1.6 to 1.7, your theme will be switched to the new default theme, called “Classic”. We therefore recommend working on your 1.7 theme before you make the switch to PrestaShop 1.7.
Some features have been dropped with PrestaShop 1.7. They were either already deprecated in 1.6, or proved too problematic to maintain.