Unspecified color variables

Would it be possible to set some appropriate values for these [color variables](https://fishshell.com/docs/current/interactive.html#variables-color):

| Variable | Description |
|----------|-------------|
| `fish_color_cwd_root` | the current working directory in the default prompt for the root user |
| `fish_color_history_current` | the current position in the history for commands like `dirh` and `cdh` |
| `fish_color_valid_path` | parameters and redirection targets that are filenames (if the file exists) |

In particular the lack of the valid path (underlined in the default theme) is noticeable.

I realized this after the switch to global variables and only setting my theme in `config.fish` using

```fish
fish_config theme choose "Catppuccin Frappe"
```

Start by reviewing the Catppuccin Frappe theme selected through `fish_config theme choose "Catppuccin Frappe"` and the linked Fish color-variable documentation. Check how the theme defines its existing variables, then assign appropriate values to the three listed variables; done means the current path, history position, and valid paths have intentional coloring, including the underlined valid path.

Written by the indexing model from the issue text.