I need fully change UI style existing application. How to tell to RichFaces that, I want use another EL CSS file for skinning? Where to put new .ecss file?
If you want to fully change the RichFaces skin you cand add this to your web.xml:
<context-param>
<param-name>org.richfaces.skin</param-name>
<param-value>skin_name</param-value>
</context-param>
and some of the skin's are: emeraldTown, blueSky, wine, japanCherry, ruby, classic, deepMarine etc.
But if you want to change something to your current(or default skin), you can do that with .ecss file.
More on this you can find here: http://docs.jboss.org/richfaces/latest_4_2_X/Developer_Guide/en-US/html/chap-Developer_Guide-Skinning_and_theming.html
Hope this helps! :)