No such subcommand: theme

[glendell03]

I can't save theme using this command fish_config theme save "Catppuccin Mocha"

[hugginsio]

Can you share the output of fish --version? You may be using an older version of fish.

[plokil]

On Ubuntu and its derivatives doing sudo apt install fish on its own will install old version of fish.
To install new fish version do:
$ sudo apt-add-repository ppa:fish-shell/release-3
$ sudo apt update
$ sudo apt install fish
And then execute fish_config theme save "Catppuccin Mocha" to apply the theme