Casper is not the default theme anymore

[tmlmt]

It is now Source, cf https://github.com/TryGhost/Casper/blob/36e886a6685f5edef6ba47cce788067e9a043233/README.md?plain=1#L3

The README file could point to this article for instance: https://ghost.org/tutorials/build-with-ghost-meet-our-new-official-theme/#meet-source-ghosts-new-default-theme

Casper is still maintained for the moment as a legacy theme used by many. It also lives in 2.8k forks including mine (see fantome) where I extended it with a few features (table of contents, locales, social media icons, etc.)