Selecting A Global Theme
To set the default theme for your visitors, login to your admin site and go to Extensions > Template Manager. Click on tapthemeipad to edit the template. There you will find a Theme select box to select your theme.
Optional: Allow Visitors to Select Theme
If you'd like to allow your visitors to pick their own theme, go to Extensions > Template Manager. Click on tapthemeipad to edit the template. Set Show Theme Selector to Yes and Save.
Now your visitors will see the theme select box in the top right hand corner.