Hi – great question!
Some themes let you add custom CSS or alternatively you could follow the steps prescribed in the Themer’s Guide to set up a tribe-events.css stylesheet – though this would load before events-admin.css you could use suitably specific rules to change the font sizes even so, such as:
#page .eventForm h4 { font-size: 3em; }
I tested the above out locally, but in the case of your theme you might change that to:
#wrapper .eventForm h4 { font-size: 3em; }
Does that help at all?