Change the current(default) theme

    Parameters
    ----------
    new : theme
        New default theme

    Returns
    -------
    out : theme
        Previous theme

def theme_set(new):
    """
    Change the current(default) theme

    Parameters
    ----------
    new : theme
        New default theme

    Returns
    -------
    out : theme
        Previous theme
    """
    if (not isinstance(new, theme) and
            not issubclass(new, theme)):
        raise PlotnineError("Expecting object to be a theme")

    out = get_option('current_theme')
    set_option('current_theme', new)
    return out