1. It stops it, so the other user's pet won't go to the pound if it is attempted.
3. The ACP-made pages are stored in the database (a.k.a phpMyAdmin) under "content"
3.5. To be frank, I 100% prefer creating new pages without the ACP. It's less clunky and adds freedom, so the old way quickly become obsolete. You can read up on it here:
link
4. It is safe to delete them, but it is best to have something to fall back on if the new theme doesn't work out. The new theme won't automatically become the new default, you have to set it as such through the ACP under settings->basic settings.
5. Not entirely sure on this one, since I don't know how you would tint/add color to the pet images
6. This can be done with the custom page solution. You can use custom style tags in them as well; just make sure you use them above the php tags!