As of version 3.3.0.GA it is not possible to ask richfaces to stop rendering CSS for borders and backgrounds. Its necessary to override each of the properties from the built in stylesheets (the ones contained in org.richfaces.renderkit.html.css which is part of richfaces-ui-3.3.0.GA.jar).
Plug and skin gives you the necessary power to do that, as would custom CSS added using the usual techniques, though plug and skin is arguably better as you can reference abstract colour definitions to compensate for missing backgrounds, like this:
<u:style name="color" skin="abstractColorName" />
The following CSS properties are useful for quickly neutralising the border and background related properties.
border: none;
background: transparent;
To get started with plug and skin you can use the following Maven2 command sequence:
mvn archetype:generate -DarchetypeGroupId=org.richfaces.cdk -DarchetypeArtifactId=maven-archetype-plug-n-skin -DarchetypeVersion=3.3.0.GA -DartifactId=fil-skins -Dpackage=com.feelitlive.richfaces -DgroupId=com.feelitlive.server -Dversion=0.0.1-SNAPSHOT -DarchetypeRepository=http://repository.jboss.com/maven2/
cd fil-skins
mvn cdk:add-skin -Dname=fil -Dpackage=com.feelitlive.richfaces.skins
This is best done from the command line as unfortunately the Eclipse plugins cannot locate the archetypes in the jboss repository (other IDEs may handle that better). You'll need to change artifactId, groupId, package, version and name properties to suit your project. Choose an artefact and package name that reflects the scope of the artefact for holding mulitple skin definitions.
It would be possible - though a lot of work - to add support for skin properties in the general format xxxBackgroundEnabled and xxxBorderEnabled to switch off rendering of those border and background CSS on a case by case basis.
You would need to work with the richfaces team to patch the xcss files in org.richfaces.renderkit.html.css. You'd also need some kind of conditional output functionality in one of the XCSS JSF tag libraries (identified by http:/jsf.exadel.com/template/util or http:/jsf.exadel.com/template in the XCSS files) which I'd anticipate using to wrap groups of CSS properties in the XCSS files.
If you made such a modification to richfaces then you would be able to disable rendering of the controversial CSS from the properties file of any skin.