Custom theme inside $fish_custom dir

Trying to create my first fish-theme I would like to create it inside the dir `~/dotfiles/fish_custom/themes/costa/` when in my `config.fish` I've specified:
```
# Path to your custom folder (default path is $FISH/custom)
set fish_custom $HOME/dotfiles/fish_custom
```

I'm a fish newbie and I don't know if that is the right way, but I've tried to change the user defined themes path.

Now I can play with my new custom themes, and backup my customizations (plugins and themes) from a single dir.