Undefined get_header function

What you downloaded was not a replacement for WordPress, it was a theme. Themes need to go inside the wp-content/themes folder. Once a theme folder is placed correctly it should appear as an option in the theme menu in WP Admin where you can preview and activate it.

In the meantime, you need to reinstall WordPress to undo the damage you caused by overwriting the root with a theme. The only time you should modify files in wp-admin, wp-includes or at the top level, are when you’re updating to a new version of WordPress ( the only exception is wp-config.php ). All customisation and additions go in themes and plugins located in the wp-content/themes and wp-content/plugins folders.

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