Hazel0017 are you at least rooted?
Alright well i'm going to assume that's a no. In order to change your theme you need to first root your phone and install a custom rom. Rooting is much like being an administrator on a computer it gives you access to change the files on the computer.
How to "root" the LG Ally - Android Forums
There are instructions on rooting it from linux and windows (8th post) just read it over first and make sure you think you can handle it. Also read the posts below such as post 19 at that link. I've never used an ally as i carry a moto droid but rooting was definately the best thing i've done regarding a phone.
in case anyone else was or is having issues with decompile and recompile of apk
i am not a fan of hex editors they are no fun and basically you follow tutorials changing the clolor hex. <~~ sucker way!!
i prefer apk manager to decompile and recompile but the latest release of apktool has a problem so when y0rk helped me figuare out the problem i zipped the exact tool i am using and having no more problems. it is still apk manager 4.9 all i did was change apktool from 1.3.2 to 1.3.1 all thanks goto the creator off the application and of course y0rk for finding the problem. the d/l link is at the bottom
have fun and good luck