If you want to get back to your old homescreen, go into the long press menu (by long pressing the screen). Press "Settings," "Themer," and "Restore Old Homescreen." You can then choose your old launcher (along with "just once" or "always"), and you'll be right back where you left off before you installed Themer.
If you want to get Themer back, run the Themer app (by pressing the Themer app icon). Go back to the Themer menu and hit the big "Set Home" button. Choose "Themer" and "Always."
We know it's a bit confusing, but these are the wonders of Android! If you have any questions, contact us at firstname.lastname@example.org.