def []=( color_tag, constants )
      @scheme[to_key(color_tag)] = constants.respond_to?(:map) ?
          constants.map { |c| to_constant(c) }.join : to_constant(constants)
    end
def []=( color_tag, constants )
      @scheme[to_key(color_tag)] = constants.respond_to?(:map) ?
          constants.map { |c| to_constant(c) }.join : to_constant(constants)
    end
Allow the scheme to be set like a Hash.