module ApplicationHelper

  def after_sign_in_path_for(resource)
    admin_path
  end

  def after_sign_out_path_for(resource)
    admin_path
  end

  def flash_messages
    return if !flash.any?
    flash.each do |type, msg|
      return content_tag :div, msg, class: "alert alert-#{type}"
    end
  end

end

module ApplicationHelper

  def after_sign_in_path_for(resource)
    admin_path
  end

  def after_sign_out_path_for(resource)
    admin_path
  end

  def flash_messages
    return if !flash.any?
    flash.each do |type, msg|
      type = 'error' if type == :alert
      return content_tag :div, msg, class: "alert alert-#{type}"
    end
  end

end

Change css class when alert
Change css class when alert
