Hello ouroumov
This is a known issue that started in the 4.15 kernel. This thread is just to let any testers know that it’s still broke in 4.16. I see rc5 came out yesterday, need to give it a try.
Are you running 18.04? I am getting ready to file a bug against the “Appearance GUI” not accepting any themes in root /themes or home /.themes, but It would be a nice touch if someone would verify this is a bug. Maybe you?
Edit
Never mind, its not a bug. A theme name change must be entered in /index.theme.