Sketch possible solution without fanstatic.

buildout.cfg

@@ -22,7 +22,7 @@ parts =
 sources-dir = develop
 auto-checkout =
     js.bootstrap
-    zmi.icons
+    zmi.styles
     Persistence
     AccessControl
     Products.BTreeFolder2

docs/ZMI.rst

@@ -10,12 +10,12 @@ browser.
 Bootstrap ZMI
 -------------
 
-Since Zope 4.0b4 the ZMI is styled using Twitter bootstrap. The previously used
+Since Zope 4.0b4 the ZMI is styled using Twitter Bootstrap. The previously used
 GIF icons were replaced by font glyphs which are stored in the package
-`zmi.icons`_
+`zmi.styles`_ together with the CSS and JavaScript needed by Twitter Bootstrap.
 
 Inside that package is a table of the `available icons`_ including the names
-which are required to use them.
+which are required to use them in the ZMI.
 
 Update packages
 +++++++++++++++
@@ -26,32 +26,53 @@ the ZMI, the default icon will be shown.
 To use one of the new icons add an attribute named `zmi_icon` to the class. The
 value should be one of the names listed on `available icons`_.
 
-.. _`zmi.icons` : https://github.com/zopefoundation/zmi.icons
-.. _`available icons` : http://htmlpreview.github.io/?https://github.com/zopefoundation/zmi.icons/blob/master/zmi/icons/resources/demo.html
+.. _`zmi.styles` : https://github.com/zopefoundation/zmi.styles
+.. _`available icons` : http://htmlpreview.github.io/?https://github.com/zopefoundation/zmi.styles/blob/master/zmi/styles/resources/zopetello/demo.html
 
-Use custom icons
-++++++++++++++++
+Use custom icons and resources
+++++++++++++++++++++++++++++++
 
-To use custom icons (which are not part of `zmi.icons`), you need to subscribe
-to ``App.interfaces.IRenderZMIEvent`` to need your resources.
+To use custom icons (which are not part of `zmi.styles`) or load custom CSS resp. JavaScript, you have to:
 
-Example from `Products.CMFCore/Products/CMFCore/zmi.py`::
+1. create a directory and fill it with your assets
+2. register this directory as resource directory
+3. register a subscription adapter for :class:`App.interfaces.ICSSPaths` resp.
+   :class:`App.interfaces.IJSPaths`. This adapter has to return an iterable of
+   paths resp. URLs which should be loaded when rendering the ZMI.
 
-    import App.interfaces
-    import cmf.icons
-    import zope.component
+Example taken from `cmf.zmiicons`_:
 
+* Register the resource directory via ZCML:
 
-    @zope.component.adapter(App.interfaces.IRenderZMIEvent)
-    def load_assets(event):
-        """Load the CMS icons for the ZMI."""
-        cmf.icons.cmf_icons.need()
+  .. code-block:: XML
 
-The subscriber is registered in
-`Products.CMFCore/Products/CMFCore/event.zcml` like this::
+      <browser:resourceDirectory
+          name="cmf.zmiicons"
+          directory="resources" />
 
-    <subscriber
-        handler=".zmi.load_assets" />
+* Create a subscription adapter returning the path to the CSS file:
 
-You can can `need` arbitrary fanstatic resources in the event subscriber, not
-only icon fonts, but also custom CSS and JavaScript.
+
+  .. code-block:: Python
+
+      import zope.component
+      import zope.interface
+
+
+      @zope.component.adapter(zope.interface.Interface)
+      def css_paths(context):
+          """Return paths to CSS files needed for the Zope 4 ZMI."""
+          return (
+              '/++resource++cmf.zmiicons/css/cmftello.css',
+          )
+
+* Register the subscriber via ZCML:
+
+  .. code-block:: XML
+
+      <subscriber
+          provides="App.interfaces.ICSSPaths"
+          factory=".css_paths" />
+
+
+.. _`cmf.zmiicons` : https://github.com/zopefoundation/cmf.zmiicons

setup.py

@@ -68,16 +68,14 @@ def _read_file(filename):
         'RestrictedPython',
         'ZConfig >= 2.9.2',
         'ZODB',
-        'fanstatic >= 1.0',
         'ipaddress',
-        'js.bootstrap >= 4.0.dev0',
         'setuptools',
         'six',
         'transaction',
         'waitress',
         'zExceptions >= 3.4',
         'z3c.pt',
-        'zmi.icons',
+        'zmi.styles',
         'zope.browser',
         'zope.browsermenu',
         'zope.browserpage >= 4.0',

sources.cfg

@@ -15,7 +15,7 @@ Persistence = git ${remotes:github}/Persistence pushurl=${remotes:github_push}/P
 RestrictedPython = git ${remotes:github}/RestrictedPython pushurl=${remotes:github_push}/RestrictedPython
 zExceptions = git ${remotes:github}/zExceptions pushurl=${remotes:github_push}/zExceptions
 zope.globalrequest = git ${remotes:github}/zope.globalrequest pushurl=${remotes:github_push}/zope.globalrequest
-zmi.icons = git ${remotes:github}/zmi.icons pushurl=${remotes:github_push}/zmi.icons
+zmi.styles = git ${remotes:github}/zmi.styles pushurl=${remotes:github_push}/zmi.styles
 
 # Required depencencies
 js.bootstrap = git https://github.com/gocept/js.bootstrap4.git branch=bootstrap4

src/App/Management.py

@@ -18,12 +18,14 @@
 from AccessControl.class_init import InitializeClass
 from AccessControl.Permissions import view_management_screens
 from App.interfaces import INavigation
-from App.interfaces import RenderZMIEvent
+from App.interfaces import ICSSPaths
+from App.interfaces import IJSPaths
 from App.special_dtml import DTMLFile
 from ExtensionClass import Base
 from six.moves.urllib.parse import quote, unquote
 from zExceptions import Redirect
 from zope.interface import implementer
+import itertools
 import zope.event
 
 try:
@@ -160,7 +162,10 @@ class Navigation(Base):
     security.declareProtected(view_management_screens, 'manage_page_header')
     def manage_page_header(self, *args, **kw):
         """manage_page_header."""
-        zope.event.notify(RenderZMIEvent())
+        kw['css_urls'] = itertools.chain(
+            *zope.component.subscribers((self,), ICSSPaths))
+        kw['js_urls'] = itertools.chain(
+            *zope.component.subscribers((self,), IJSPaths))
         return self._manage_page_header(*args, **kw)
 
     security.declarePublic('manage_zmi_logout')

src/App/dtml/manage_page_header.dtml

@@ -7,6 +7,12 @@
 <meta http-equiv="content-type" content="text/html;charset=&dtml-management_page_charset;" />
 <dtml-call "RESPONSE and RESPONSE.setHeader('content-type','text/html;charset='+management_page_charset)">
 <title><dtml-if title_or_id><dtml-var title_or_id><dtml-else>Zope</dtml-if></title>
+<dtml-in css_urls>
+  <link rel="stylesheet" type="text/css" href="&dtml-sequence-item;" />
+</dtml-in>
+<dtml-in js_urls>
+  <script type="text/javascript" src="&dtml-sequence-item;"></script>
+</dtml-in>
 </head>
 <body>
   <div class="container-fluid">

src/App/interfaces.py

@@ -15,7 +15,6 @@
 
 from zope.interface import Attribute
 from zope.interface import Interface
-from zope.interface import implementer
 
 
 class INavigation(Interface):
@@ -31,10 +30,9 @@ def manage_zmi_logout(REQUEST, RESPONSE):
         """Logout current user"""
 
 
-class IRenderZMIEvent(Interface):
-    """ZMI is rendered."""
+class ICSSPaths(Interface):
+    """Paths to CSS resources needed for the ZMI."""
 
 
-@implementer(IRenderZMIEvent)
-class RenderZMIEvent(object):
-    """Event fired when the ZMI is rendered."""
+class IJSPaths(Interface):
+    """Paths to JAvaScript resources needed for the ZMI."""

src/Zope2/App/configure.zcml

@@ -17,7 +17,6 @@
   <securityPolicy
       component="AccessControl.security.SecurityPolicy" />
 
-  <subscriber
-      handler=".zmi.load_assets" />
+  <include package="zmi.styles" />
 
 </configure>