document how to add body classes in classic UI

Author: jensens

bodyclasses.md

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+# Add custom classes to the `body` element
+
+Body classes are generated in the ``LayoutPolicy.bodyClass` method in mdoule `plone.app.layout.globals.layout`.
+Contained is a feature to plugin in own body-classes using named adapters.
+
+Create a class like so:
+
+```Python:
+from plone.app.layout.globals.interfaces import IBodyClassAdapter
+
+@implementer(IBodyClassAdapter)
+class CustomBodyClasses(object):
+    """Additional body classes adapter."""
+    def __init__(self, context, request):
+        self.context = context
+        self.request = request
+
+    def get_classes(self, template, view):
+        return ["additional-class", "another-css-class"]
+
+```
+
+Then register the adapter in ZCML:
+
+```XML
+<adapteer
+    factory=".custombodyclasses.CustomBodyClasses"
+    for="* *"
+    name="myproject-customclasses"
+/>
+```