add overrides examples to layers.md

Author: toalba

docs/classic-ui/layers.md

@@ -58,13 +58,54 @@ Then views and viewlets from your product can be enabled on the site instance us
 ### Unconditional overrides
 
 If you want to override a view or a viewlet unconditionally for all sites without the add-on product installer support, you need to use `overrides.zcml`.
+You can override classes and templates in this file.
 
-```{todo}
-Explain how to use an `overrides.zcml`.
+Overide a class:
 
-See https://github.com/plone/documentation/issues/1426
+```xml
+<browser:viewlet
+    name="plone.site_actions"
+    manager="plone.app.layout.viewlets.interfaces.IPortalHeader"
+    class=".yourViewlet.YourClassViewlet"
+    permission="zope2.View"
+    layer="your.app.interfaces.IYourAppLayer"
+/>
+```
+
+Overide a template:
+
+```xml
+<browser:viewlet
+    name="plone.logo"
+    for="*"
+    template="your-logo.pt"
+    permission="zope2.View"
+    layer="your.app.interfaces.IYourAppLayer"
+/>
+```
+You can combine both as well. For example, you can override a class and use a different template for it:
+    
+```xml
+<browser:viewlet
+    name="plone.site_actions"
+    manager="plone.app.layout.viewlets.interfaces.IPortalHeader"
+    class=".yourViewlet.YourClassViewlet"
+    template="your-logo.pt"
+    permission="zope2.View"
+    layer="your.app.interfaces.IYourAppLayer"
+/>
 ```
 
+If you want to override a viewlet or view of a specific add-on product, you need to use enheritance the Layer interface of the add-on product into your own Layer interface.
+For example, if you want to override the `sidebar` viewlet of the `collective.sidebar` add-on product, you need to create a new Layer interface and inherit from the `collective.sidebar` Layer interface.
+
+```python
+from collective.sidebar.interfaces import ICollectiveSidebarLayer
+
+class IYourAppLayer(ICollectiveSidebarLayer):
+    """Marker interface that defines a browser layer."""
+
+```
 
 (classic-ui-creating-a-layer-label)=