This tutorial provides the steps required for setting default theme in Keynote on Mac OS X Lion. Whenever you click File -> New option, the following Choose Theme window will be displayed and you can select a theme from the below list.
If you are planning to use one standard theme for all your presentation then you can set that as the default theme and avoid the step of choosing a theme.
Click the Keynote menu and select Preferences from the menu list.
In the General Preferences window, navigate to For New Documents and set the radio option as Use theme and select your preferred theme.
Now when ever you open a new presentation, the theme will be automatically set to your default theme i.e Editorial.