As I have purchased the theme, my question is answered - it does NOT support custom grid layouts. Just the freeform grid. Disappointing.

This theme uses the foundation 5 grid. You would need to utilise the custom page type to get this to work properly.

This is great news! I am having difficulty locating the "custom page type"
Do you maybe mean "basic page type"?
I have enclosed a screen grab of my "page type"
Thanks!
Arlene

I believe the 'blank' page type is indeed the one you need.

Okay anybody who looks at this - grid layout support is a fantastic tool for difficult or complex content. Not all the themes offered for 5.7 support it yet, but Equinox does.

To take advantage of this you must choose a "blank" page template, and a "blank" page type. (see C5 documentation)
If you have sample content installed and see the text running from one side to the other unconstrained, don't get scared. Select the main area tab and "add layout" - the Foundation grid system is installed and ready to go.
Thank you C5hub!