Don't fret, this is an easy fix. Open Themer back up, and access the Themer Menu. You can get there by long pressing the homescreen and tapping Themes. Once you're back at the Themer Menu, you should see an option marked SET HOME. Tap on this option, select Themer, and press "Always." Now, any time you press Back or Home, you will return to your awesome new Themer homescreen!
If you have any trouble, or just have a question, please contact firstname.lastname@example.org