Converting old website into localhost website

From my backups found an old website, which have been unused since 2017. Just for testing I had to make a database and user and the import raw database via command line for that user.

After that I deleded wp-config.php file and run (re) installer, which made a new wp-config.php file.

I also had to edit wp-config.php file for converting the site address

After that wp-login.php worked, but I had forgotten user password, so I had to reset the password via phpMyAdmin.

An another tutorial can be found from also

After resetting the password I was able to log in and the site worked. I disabled all plugins and installed default Twenty Ten WP Theme

Site worked sill, but when I upgraded WP to the version 5.0.2 by copying wp-admin and wp-includes folders over the old ones, admin dasboard worked, but site got a grey or white death, depending on theme, so I downgraded WP to the version 4.7, and site worked again.

Just out of curiosity, what causes these grey or white deaths?