A) From Property View
Right-click the root package in the structure tree and then select [Set Font]
B) From Property View
Select the root package in the structure tree and then go to its Property view (The bottom left pane) and then click the Default Font button.
When the Font chooser box first opens, all of the top check boxes are ticked with the default setting. Uncheck them in order to choose your preferred font style. Also checking [All Available Font Families] box will show all the font styles that you can select.
If you are using the free edition, download free trial of Astah Professional to try it out.