How can I customize the active admin layout?

The active admin layout is not actually defined as a layout file, but is generated programatically. Placing a custom layout in the layout directory will therefore not actually override the default layout.

You can, however, monkey-patch or duck-punch the active admin layout methods inside your application.

The following will add an ie-specific stylesheet to the header:

module ActiveAdmin
  module Views
    module Pages
      class Base < Arbre::HTML::Document

        alias_method :original_build_active_admin_head, :build_active_admin_head unless method_defined?(:original_build_active_admin_head)

        def build_active_admin_head
          within @head do
            meta :"http-equiv" => "Content-type", :content => "text/html; charset=utf-8"
            insert_tag Arbre::HTML::Title, [title, active_admin_application.site_title].join(" | ")
            active_admin_application.stylesheets.each do |path|
              link :href => stylesheet_path(path), :media => "screen", :rel => "stylesheet", :type => "text/css"
            end
            active_admin_application.javascripts.each do |path|
              script :src => javascript_path(path), :type => "text/javascript"
            end
            text_node csrf_meta_tag
            text_node "<!--[if lt IE 7]>
            <link rel=\"stylesheet\" type=\"text/css\" media=\"all\" href=\"admin_ie7.css\ />
            <![endif] -->".html_safe
          end
        end

      end
    end
  end
end

Clearly an ugly solution.

When a view is defined in a gem AND in the rails app, the one defined in the Rails app is served. It's a logic priority.

So if you need to override all or some active admin views, you'll have to copy these in your app and change them as you desire.