The text can be edited in step 3 "Extras" of the event form.
In the "Do you want to limit this event's online capacity?" make sure "Limit online capacity" is checked and the field "Maximum capacity message" will appear with a field to edit the text.

The color and font family of this text can not be edited in fine tuning tab or in Online Express in general.  This information is controlled in the hosting website and either has to be changed there or changed by targeting the "bboxevent_BBCapacityMessage" element. Please consult with a web developer for these changes as support can not assist with 3rd party sites.