My site's banner image has the following text written over it:
Where can I edit this text to customize it for my site?
Studio -> Pages -> System -> HomePage -> Settings -> Cover
That should be it, as I'm not 100% sure because I shut mine off.
I checked and it was not found there, attached is screenshot of the Settings -> Cover dialog.
Any text can be changed via Studio > Polyglot, just search for a part of the text (like "Unity is strength"), then edit it.
That worked. Attached is screenshot showing.