Add ability to warn on missing theme values (#3705)

This PR adds the ability to warn in the `theme_importer` when a theme is
missing values.

Providing the `--warn-on-missing` flag to the `theme_importer` will
print a warning for missing theme value when printing the theme.

```sh
cargo run -p theme_importer -- --warn-on-missing
```

Release Notes:

- N/A