Trying To Edit Styling Of A Client Gallery

I've made a copy of my normal theme and trying something out in a test gallery.

I'm trying to edit the gap between image thumbnails and also the styling of the little popup that appears when I hover over an image (the one with cart icon, download icon and filename).

I cannot find where to do any of this.  I've tried every setting I can find in the theme editor (and yes, I'm on the correct theme).  

I've attached a screen grab.  What layout is this and where do I edit it?  It looks similar to 'on photo' but changing those settings doesn't affect the result.  If I change background colour etc within the theme editor then the changes are visible so I'm in the right place, it's just the actual thumbnail grid itself which eludes me.

Any time I try to change anything like this I just get completely lost.

Screen grab

What is the link to the page? I can't tell anything from the screenshot.

https://robandsarahgillespie.co.uk/client/demo-gal...

No password

Go to the Photos tab of that gallery in the admin and click the Photo Display Settings option. 

Adjust the spacing under the "Spacing (margin)" option.

The color behind the icons and file name that shows in Design -> Edit My Theme -> On Photo -> Text Background at the top of the page.

Thanks, as always.

I get lost in all the settings sometimes (which is a good thing).

Is your entire website made in Sytist?  Very nice!!!

Thanks, but, no, it's on Wordpress.  Sytist is just for the client galleries.