This is just something I threw together as an App Store, so it probably won't be too good. I didn't have the time available to integrate it into the OS itself, but it shouldn't be very hard at all to do so. If you want me to do it, let me know.
All me. You can change the shop icon at the start if you like. Also, the apps and the popups have 6 costumes (I didn't know how many apps there were- some could be built in?) so they can be changed according to the apps. PRESS THE DOWNLOAD BUTTON TWICE!!! I can't figure out why that doesn't work, if someone could, that'd be great. It's probably a bit buggy still, but chances are that's Scratch 3 being annoying. - Changed it so that the download button doesn't show after an app has been 'downloaded'.