From given theme name, return theme file path from
        ``settings.CODEMIRROR_THEMES`` map.

        Arguments:
            name (string): Theme name.

        Raises:
            KeyError: When given name does not exist in
                ``settings.CODEMIRROR_THEMES``.

        Returns:
            string: Theme file path.

def resolve_theme(self, name):
        """
        From given theme name, return theme file path from
        ``settings.CODEMIRROR_THEMES`` map.

        Arguments:
            name (string): Theme name.

        Raises:
            KeyError: When given name does not exist in
                ``settings.CODEMIRROR_THEMES``.

        Returns:
            string: Theme file path.
        """
        if name not in settings.CODEMIRROR_THEMES:
            msg = ("Given theme name '{}' does not exists in "
                   "'settings.CODEMIRROR_THEMES'.")
            raise UnknowThemeError(msg.format(name))

        return settings.CODEMIRROR_THEMES.get(name)