The following error may occur in the Sitecore log files when starting the Sitecore App Center application:
ERROR Cannot load icon file 'Custom/16x16/appcenter.png'. Zip archive '/sitecore/shell/Themes/standard/Custom.zip' with icons not found.
This behavior does not affect application's functionality, while only logs an error message.
Copy the Custom.zip file to the /Website/sitecore/shell/Themes/Standard folder.