I would suggest you read all you can about ENVIRONMENT in the online doc.
Then you read it again...
I would suggest you add icons, and you do not modify the existing ones.
Then you find default catia icons in the install folder somewhere...resources\graphic\icons. You can look at them, use some to create new ones...
This folder is pointed by your ENVIRONMENT file under the name: CATGraphicPath
So you define a folder somewhere where you put your entire customized ENV file like C:\MYENV then you create the folders …resources\graphic\icons and you put you customized icons file there.
In your CATGraphicPath you add your path ...C:\MYENV\resources\graphic\icons at the beginning so your icons will be first on the icons dialog box.
Define all other files you need for your ENVIRONMENT.
Start CATIA using your ENVIRONMENT, use icons as you want. Close CATIA.
Delete your users CATSettings, and start CATIA as a user and check everything is OK.
Eric N.
indocti discant et ament meminisse periti