origin xcss

<u:selector name="body .blocAuthentificationInterieure">
	<u:style name="background-image">
          <f:resource f:key="SphinxGradient">
              <f:attribute name="gradientHeight" value="100px"/>
              <f:attribute name="height" value="150px"/>
              <f:attribute name="gradientColor" skin="blocConnexionInterieureBackgroundColor"/>
          </f:resource>
	</u:style>
	<u:style name="background-repeat" value="repeat"/>
	<u:style name="background-position" value="0 50%"/>
</u:selector>

migrated to

body .blocAuthentificationInterieure{
background-image:"url(#{resource['SphinxGradient?gradientHeight=100px&height=150px&gradientColor=Skin.blocConnexionInterieureBackgroundColor']})";
background-repeat:repeat;
background-position:0 70%;
}

Possible solution :

add in resource-mappings :
com.natixis.sphinx.ce.internet.images:blocAuthentificationInterieureBackGroundImage.png=org.richfaces.renderkit.html.BaseGradient
{gradientHeight=100, height=150, gradientColorParam=blocConnexionInterieureBackgroundColor'}

and then in the ecss :

background-image:"url(#{resource['com.natixis.sphinx.ce.internet.images:blocAuthentificationInterieureBackGroundImage.png']})";