Hi and welcome to grav @kilowahn
A good place to start is generally to get a look and feel of template is direct from the skeltons section, but with grav being open source, a lot of the templates are created by developers.
The skeleton generally will be a out of the box, install solution which includes demo pages.
The theme is basically for people to install a clean copy of grav and then add the theme manually, since grav is developing, it will show flaws in the template, but can be easier to identify and fix, and get you in to “the inner workings” of grav.
You can always set it up like this (grav + theme) and then copy the pages folder and copy it in to your “new” grav site with the activated theme and see the magic.
It is always worth a google with the phrase “[template name] grav github” as you will find info and find out if the developer is supporting their theme.
tranduyhung/grav-theme-big-picture: Big Picture theme is a port of Big Picture by HTML5 UP for Grav CMS (github.com)
This will take you to the developer, I noticed he has updated a blueprint a couple of months ago and people are answering questions.
Saying that in the readme it does state.
Big Picture theme is a port of Big Picture by HTML5 UP.
The original html5up looks like it has been updated on top of that.
So checking html5up.net might also give you a bit of extra info but the documentation and upkeep will always be down to the developer and not grav. You can get good help with particular issue on the forum or in the chat room, the developers github and a bit of googling.
Always worth checking the templates\js folder, for the names of included animation source
In this case you have
poptrox - ajlkn/jquery.poptrox: A lightbox gallery plugin for jQuery (github.com)
scrollex - jQuery Plugin To Animate Content As User Scrolls - scrollex | Free jQuery Plugins (jqueryscript.net)
scrolley - jQuery Scrolly Plugin Demo - All In One (jqueryscript.net)
skel.io - ajlkn/skel: A lightweight responsive framework for the www (github.com) but has stopped being devolped and the author has made a new more web compliant ajlkn/responsive-tools: Make responsive design less annoying (github.com)
pamtbaau beat me to it in his reply, whilst I was compiling this doc, but hopefully it will give you insights on how to get more out the template.
Developers also hate writing documentation lol .