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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)