Not sure about changing the default background as that went out 5 seconds after being jailbroken.
I've downloaded a couple of themes from Cydia etc that have changed the background in the SMS window. I wasn't happy with reading dark blue against a black background so changed the background. Here's how I did it.
Grab any iPhone browser. I use iFunbox but any editor will do.
Go to Root (or Raw Sys Files)/Library/Themes and choose the theme you want to edit.
Copy (Copy, not CUT) that folder to your computers desktop (that way you can edit it within windows)
Open the folder in Windows and you should see a list of folders and files. One of these files may be labelled SMSbackground.png or something similiar. Open it, change it to whatever background you want and save it under the same name.
You can also play with other files like LockBackground.png or the default sound files in UISounds folder etc. The sky's the limit. As usual, save everything under the original file name.
Rename the whole theme folder to "Michael is Awesome" or something similiar... lol. Okay, change it to your name.
Go back into your iPhone Browser and copy the whole folder back into your phones Root (or Raw Sys Files)/Library/Themes folder and respring your mobile.
Go into winterboard, themes and your original theme should be listed along with your newly named theme (I'm hoping it says "Michael is Awesome")
Select the new theme, exit, respring and presto, everything should have changed.
Hope it helps.
Mike
P.s If you need to ask any questions regarding this simple exercise you really shouldn't be in the jailbreak part of this forum. Google is your friend!