The css element allows to specify the location and name of a CSS stylesheet. It is of course allowed to have several css elements in the same configuration, provided that all css elements but one have an alternate="true" attribute. All these CSS stylesheets are listed at the end of the View menu. The CSS stylesheet which has an alternate="false" attribute is the one which, by default, is used to style the document being opened.

Excerpts from rng_section_config/common.incl (same file for all variants):

<css location="section.css" name="Normal" />