The mystery of the two unknowns: the site was handed over to the client, fully operational, there were no complaints.
After some time, the site stopped displaying correctly, which the client complained to me about. After digging around inside, I realized a simple truth: some security settings have changed on the hosting. And now, when trying to upload a style file, the client sees a 403 error:

It turns out that it's not my fault. Hosting also tried its best. But the site is not working.
Who is to blame?