1.: yes, you can change (some_ stuff on https://your.gitlab.install/admin/appearance (also available via the admin menu). Also, every user can choose their own dashboard in the user’s settings.
2. Changing code without being able to remove? Not sure what you mean by that. And the code is versioned, so you can get it back. Also, you can add them as developers and require a merge request before merging, so you can review their changes