custom_mailer_themes.rb

module ActionMailer
  class Base

    # call deliver! and then clear the theme out
    # need to clear the theme on both success and failure do to errors
    def deliver_with_clear_theme_path!(mail = @mail)
      begin
        deliver_without_clear_theme_path!
      ensure
        clear_theme_path
      end
    end
    alias_method_chain :deliver!, :clear_theme_path

    protected

    # Allows us to use a custom mailer template for a theme
    def set_theme(theme)
      # make sure no left over theme paths are in the view paths
      self.class.view_paths.delete_if{|dir_path| dir_path.path.match("themes")}
      @theme_path = File.join(RAILS_ROOT, 'app', 'themes', theme, 'views')
      self.class.view_paths.unshift(@theme_path) if File.exists?(@theme_path)
    end
    
    # Delete the theme path off of the view_paths after the emails is sent
    # The view paths are cached so if 1 company has a custom theme template and the next company does not, 
    # that company would get the previous company's themed template instead of the default, unless that path is cleared out
    def clear_theme_path
      if @theme_path && self.class.view_paths.first == @theme_path
        self.class.view_paths.shift
        @theme_path = nil
      end
    end

  end
end

some_mailer.rb

class SomeMailer < ActionMailer::Base
  
  def contact(company)
    set_theme(company.theme)
    # normal mailer stuff here ...
  end

end