" You can use the Default Site option in conjunction with the site page block “List of sites” to create a branded site index and to add an about page for the overall installation."
[A Clue, no details though (and the link there is broken)]
How can I add a logo and some text to the default “List of Sites” Omeka-S landing page?
Based on this, I am starting to think that the baked-in, default landing page of Omeka-S cannot be customized. Rather. one must create a new Site that consists of this List of Sites Block, then promote it to the front, i.e., make it the new Landing Page, replacing what’s there?