When a CSS document contains imports to other CSS documents, the references are also passed through the XML catalog URI mappings in order to determine an indirect CSS referenced location.
@import "http://host/path/to/location/custom.css";and then add your own XML catalog file that maps the location to a custom CSS in the XML / XML Catalog preferences page:
<uri name="http://host/path/to/location/custom.css" uri="path/to/custom.css"/>
<uri name="http://www.oxygenxml.com/extensions/author/css/userCustom.css" uri="path/to/custom.css"/>
This extra mapped CSS location will be parsed every time the application processes the CSS stylesheets used to render the opened XML document in the visual Author editing mode. This allows your custom CSS to be used without the need to modify all other CSS stylesheets contributed in the document type configuration.