Drag the custom icons you want to use to the window in the ‘Get Specified Finder Items’ action.

To add the Resources folder (where you will copy the custom icons to) to the ‘Copy Finder Items’ action, right click on the app you want to modify the icons of & select ‘Show Package Contents’. Open the Contents folder to find the Resources folder & then drag this from the Finder window to the location drop down list in the ‘Copy Finder Items’ action

Save the workflow as an application, then any time the app is updated run the Automator app to change the icons.

To automate it even further you can set up rules in Hazel to automatically change the icons once the app has been updated without having to manually run the workflow yourself.