There is a "CSS Class" parameter which can be set for Service Catalog form components, this can be used to apply CSS formatting to the contents if form fields. The CSS formatting comes from the file %USM_HOME%\view\webapps\usm\gwt\fdBase\css\formdesigner.css. You can edit this file and add your own CSS format definitions and apply them to fields.
i.e. Add a definition similar to the following at the end of the file:
(After doing this, you must clear the browser cache for this to take effect).
If you edit a text field on a form and enter "cssTest" in the "CSS Class" attribute of the field, the contents of the text field will then be in bold.