What System Menu where you on? If it was lower than 4.2 (highly doubtful considering you say you bought the Wii only a week ago), you could use SaveMiiFrii to recover.
If you were on 4.2, you should be able to use SaveMiiFrii to recover your Wii once there is a game disc with 4.3 on it.
If you were on 4.3 (I highly doubt you were, seeing as you could install themes), you'll have to wait a LONG time to be able to fix it, or else get a modchip.
Anyway, SaveMiiFrii works like this: Get a GC controller, plug it into the Wii (may need to be in a certain slot, not sure) and hold down all four directions on the d-pad and turn on Wii. You should hopefully get a black screen with the System Menu version in the lower right corner. Note that on official GC controllers, and most unofficial ones too, you will need to take controller apart to press down all four directions at once.
If you don't have a GC controller, or don't want to take one apart, order a SaveMii from [
www.savemii.net], which is a device that slots into a GC memory card slot and does the same job as SaveMiiFrii. Note that a GC controller would probably be cheaper and you could then do SaveMiiFrii method.
Once you are at that black screen (known as "Recovery Menu"), if you insert a disc with a higher System Menu than the currently installed one, it will install the update from the disc. So if you're on 4.1, inserting a disc with 4.2 would update the Wii, removing the bad theme. However, if you are on 4.2, you will need to wait until a disc comes out with 4.3 on it (perhaps Metroid:Other M will have it?)
Edited 1 time(s). Last edit at 07/02/2010 02:30PM by SifJar.