My solution for add a css suffix for the module:

Open modules/mod_iccalendar/mod_iccalendar.php

Locate the line

defined( '_JEXEC' ) or die( 'Restricted access' );

After this line add in a new line:

$moduleclass_sfx = $params->get('moduleclass_sfx','');

Now open modules/mod_iccalendar/mod_iccalendar.xml

Go to line 204 and add

<field name="moduleclass_sfx" type="text" label="COM_MODULES_FIELD_MODULECLASS_SFX_LABEL"
description="COM_MODULES_FIELD_MODULECLASS_SFX_DESC" />

Now in advanced options tab you can find the new option.

Please, include this code in the new version of iCalendar.

(Sorry for my language, english isn´t my mother language)

Hi Crokan !

Css class suffix for the module will be integrated into the next version of iCagenda.
Thank you for your tip for those who do not want to wait.