You'll find Shadow & Light options under it's own configuration category in ElvUI's config section by typing /ec Shadow & Light does not disable or hinder the normal function of ElvUI but does add additional features that are not yet implemented or will not be added to ElvUI.
For technical support reasons and other reasons as well, we have moved to a combined effort on this external edit of the ElvUI. Originally this project started as two personal edits of ElvUI by Darth Predator and I (Repooc). This addon can not be used without ElvUI installed and enabled. It also means if we push an update it will not mess with any ElvUI files.
It means that any time ElvUI gets updated, you will not have to worry about it overwriting our edited version of ElvUI. Shadow & Light is an external edit of ElvUI, which means it doesn't change ElvUI's files directly.