Share your experience!
My child was playing with my phone and changed the background by accident. I go back to try to change it back to the original preloaded one, but I can't find it! I have something similar in a bright purple, red, orange...all the colors except for the original blue. It looks something like this: http://www.techcredo.com/android/sony-ericsson-xperia-arc-preloaded-wallpapers/attachment/se-xperia-...
How can I get it back?
The default background is included in one of the themes. Tap the menu button at the home screen, select settings and then the theme called Flow. If it's already selected, select another one and then switch back to Flow.
I don't have a theme called Flow. All I have under themes are: Emerald, Silk, Gold, Sapphire, Amethyst, Turquoise, and Ruby. And they all look like: http://www.sparkyhub.com/wp-content/uploads/2012/05/sony-xperia-got-android-4-0-update-first-review-... but in different colors.
Thats sounds strange. The preinstalled backgrounds and themes can't be deleted from the phone unless you root the phone.
The name of the themes might differ depending on what software version you have. Have you checked all the themes? You should have a blue one.
If you can't find the blue theme, try updating the phones software using a computer and the USB-cable.
I already have the most recent update. I definitely could not delete any of the reinstalled themes and backgrounds as I have no idea how to root the phone. The only blue-ish theme I have is called Sapphire, but when I select it, the background is of the blue squiggly line. Are there multiple backgrounds within a theme that I can switch to?
Try this:
Long press the main screen (like when you add a widget)
select Wallpapers
then select Xperia(TM) wallpapers
This brings up the original background list from gingerbread, even on my ics mini pro.
I hope you will find them!
Edit: now I see that I have all the old backgrounds, except the blue one. Sorry 
Maybe you have 4.0.4 wherein the default SE Themes where replaced with SONY themes which is cosmic flow. The previous default themes are not anymore available for ICS. It is only for GB
Ohmigosh! That must be it! I do have 4.0.4. Argh! Is there any way to go back to the previous version?
If you contact a local service location they should be able to downgrade the software in your phone. It's however not possible with PC Companion or Update Service.