After Upgrading from D7 to D8, D8 does not look like D7 at all
Drupal 8 is very different from Drupal 7, same goes with previous major versions of Drupal.

This means that themes and modules can only work for a single Drupal major version (fx 6, 7 or 8).

When you migrate a site from D7 to D8 you have to re-implement the theme. In Drupal 8 we now use Twig as the template language so you pretty much have to start from scratch (unlike 5 -> 6 -> 7 where the theme differences wasn't that big and a lot could be reused).