Figured out how to theme the Appstore icons in Settings.
Some icons will theme automatically with whatever you themed their larger Springboard icons with Winterboard. For me, it was Converter, iMovie, and TheDeep (pinball game). These were already themed.
Other apps will use the smaller (icon.png) file as opposed to the larger (icon@2x.png) for Springboard.
For these apps just add the icon.png image to their bundle's folder in Winterboard.
For me, these apps are Netflix and Pool.
The odd one out was SoundHound which had it's own icon for Settings named
icon-29@2x.png.
So basically, if you want to change the Appstore Settings icons, use iFile and look the the app's folder. You will have to play around with the different icon names until you find one that works.
Note: some icons didn't change for me even with the correct name, I had to set Summerboard Mode (thru Winterboard) and respring for them to show. Once shown then disable Summerboard mode. Toggling thru Summerboard mode seems to clear the cache of the older images.
Hopefully this helps anyone trying to change the Appstore icons.