The UIImages folder will have:
AppIconMask.png
AppIconOverlay.png
AppIconShadow.png
If you are doing Cydia then you will have to download try the theme. However, if you manually access your phone you can just look for those files.
Also, check MacThemes and go for the theme threads which have a huge number of pages.
Remember, if you are using ToDo, but the theme only has an icon for Things, you can just rename the icon to ToDo.png and be fine. So you can repurpose icons you have, but aren't using.
PS. Dealing with themes is significantly easier if you manually upload files to your phone.