Go to the specific Themes folder in iFile (/var/stash/Themes). Each/any icon that is themed should be under the folder titled "Bundles." Once you open the Bundle folder, you should see a whole bunch of folders, which are labeled by the bundle identifier name of the app (for example, the Mail app will have a folder titled "com.apple.mobilemail").
All App Store apps will be listed under Applications (in the left panel of iFile with the iPad in landscape mode). You'll have to look through them to find how Apple has named the icon. Or, you can get a free tweak from Cydia called AppInfo, which will display (usually) the name of the icon (and the app's Bundle ID).
The Cydia apps are all the way back in iFile (/Applications). You can look at them also to get the icon names and Bundle IDs.
Hope some of this helps.
Marilyn