Can't install theme based on Classy

I’m using 8.5.3, and trying to follow the guide for creating a classy subtheme at

I have created a sub-theme of classy, exactly as described, but with a different name. It doesn’t appear in admin/appearance.

Here’s the tree of my theme:

├── [drwxrwxr-x 4.0K]  dischord
│   ├── [drwxrwxr-x 4.0K]  css
│   │   └── [-rw-rw-r--    0]  style.css
│   ├── [-rw-rw-r--  463]
│   └── [-rw-rw-r--  116]  dischord.libraries.yaml
└── [-rw-rw-r-- 1.2K]  README.txt

Classy does not appear in admin/appearance either – should it?

I’ve tried multiple times copying the text from that guide into new directories in themes/, including without changing anything at all (e.g. leaving it as ‘fluffiness’). None of these themes appear.

When I install a theme (like pixture_reloaded) it does go into the themes/ folder, and it does appear on the admin/appearances page. I’ve tried comparing the permissions and the file contents, and I can’t really see what else I could be missing…

Drupal version: