Rather than constantly restarting the server, I'd recommend that you a browser tool such as Firebug to dynamically update the CSS to create and update selector rules and then apply them into the associated files once you're satisfied with the changes. In terms of actual files, the main CSS file for the calendar can be found in /Slingshot/source/web/jquery/fullcalendar/fullcalendar.css and each (Alfresco Share) theme will have a presentation.css file found in <app-server>/webapps/share/themes/<theme-name>