ZCML File
configure.zcml

<configure
    xmlns="http://namespaces.zope.org/zope"
    xmlns:browser="http://namespaces.zope.org/browser"
    xmlns:zcml="http://namespaces.zope.org/zcml"
    >
<browser:layer
    name="css"
    interface="zope.app.css.layer"
    bbb_aware="true"
    zcml:condition="have deprecatedlayers"
    />
<interface
    interface="zope.app.css.CSS"
    type="zope.publisher.interfaces.browser.IBrowserSkinType"
    name="CSS"
    />
<browser:resource
    layer="zope.app.css.layer"
    name="zope3.css"
    file="zope3.css"
    />
<browser:resource
    layer="zope.app.css.layer"
    name="skin.css"
    file="skin.css"
    />
</configure>