Why is Drupal not seeing the theme?

Question

Drupal

Good day
I have a ready project. I uploaded the database, uploaded the site files, everything is ok, the site has opened. But as soon as I entered the admin panel along the path site_address/user, the theme disappeared, that is, already at the authorization stage, I saw only html markup without styles. Same thing with the site. Not only does it not see the topic, it also now displays only the Main, site name and "exit", in general, a bare header. If you go into the structure - blocks, at first it showed a message that such and such a block was disabled. he was given the wrong region or region, then it disappeared, but all disconnected blocks hang.
The directory structure for the site is "root folder/sites/site_folder/themes/my_theme_name"
This theme is not in the admin panel, although when I went to Blocks for the first time, I saw a tab with the name of my theme (all blocks are disabled on it), but after enabling and disabling standard themes, it disappeared.
Question. Why was the topic visible at first, but disappeared after authorization? How to properly connect this theme to the site so that it is in the admin panel and displayed correctly?
Before uploading the site, I cleaned all tables with a cache in the database.
There are no errors in the reports, except that some items need to be updated.