After Upgrading from D7 to D8, D8 does not look like D7 at all I have successfully performed a D7 -> D8 migration of my company site. Everything seemed to go well, but when I'm accessing the Drupal 8 site, the entire design is pretty much gone.

How can I get the old design back again for Drupal 8?
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).