No, the best way to do this would be via a custom widget -- you could do this fairly easily, I think, by copying the existing FileUpload widget (/tomcat_folder/webapps/Thingworx/Common/thingworx/widgets/fileupload) and modifying the fileupload.runtime.css to your desire. The you would just need to rename the widget in the appropriate places to something like newfileupload and import it into the platform.
