css_files
A comma separated list of Cascading Stylesheet (CSS) files to be applied to the layout. These files will be applied the file defined in theme_css_filename This property is designed to allow for special circumstances when one stylesheet isn't enough.
Refer to Cascading Style Sheets (CSS) and the Style weblet for more information on the use of CSS in your layouts.
Default value
''Valid values
A comma separated list of path and CSS filename values, relative to the style virtual directory, enclosed in single quotes.