The Stone Steps version of the Webalizer produces all reports with a reference to a style sheet document named webalizer.css, describing the layout of the generated reports. The 'CSS directory' parameter specifies the URL path to the location of the webalizer.css file.
If you copied the webalizer.css file to the output directory for the generated reports, you can leave the field blank or add './'. However if you enabled the 'clear output directory' option or want to use one style sheet for several reports, you can specify the virtual directory of the website hosting reports here (e.g. '../css/').
Please note: Only the Stone Steps version of the Webalizer is using style sheets for the layout of the generated reports, so the other Webalizer versions are not able to use this parameter.