The Decorous template does not have this file. Is there another way to change that spinning icon from UNA to our own?
You may copy this file from Protean to Decorous. Just put it to modules\boonex\decorous\data\template\system\ folder and then edit as you need. After the clearing of your cache it will work.
Will updates overwrite this if the site is set to ‘forced updates?
You placed the new file there, so it will not be touched by the upgrade.