7.1 page sections oddities, header, background image

Site URL: https://thevoterparty.org/

Hi I admit I'm feeling a little foolish and out of my depth, but it seems like forum users are super helpful and I'm not ashamed to ask for help. I took a job to help fix content management and layout issues and didn't realize how massively different 7.1 is.

We have a page that someone - as far as I know, not me - edited... that looks the way the site owner wants it to look. But I can't figure out how to recreate the look elsewhere on the site. https://thevoterparty.org

When I copy the page, it doesn't copy the layout elsewhere. I wonder if that's because there's custom CSS causing the layout to do different things; but the point is I like it and I want to leverage it throughout, but I'm afraid I don't know quite how.

https://thevoterparty.org/ is pretty much right  - at least the header seems to be. In desktop mode, it looks cool, with that background image running under the header. Q1: can I get that design from this page, with a rainbow image forming the background of the header, across every page on the site? 

https://thevoterparty.org/onepagedoesn't do that and I can't seem to make any other pages do it either. But /onepage does have a gif the project manager would like to see elsewhere on the site, because the team likes the animated bit. Q2: Is it possible to set the animated gif on /onepage to basically be the background image of the global site header? basically mashing together the setup of the current homepage, with the image from that internal /onepage page? 

All of this supposes making the mobile version of the site work as well. When I look at the mobile version of https://thevoterparty.org I think it's where we want it. So if making the rainbow graphic shift into the header the way we hope ALSO causes the mobile view to get messed up, I'll have to think again. 

Thanks in advance for help! 

edit: this is kind of a post script to what I mean by being confused by 7.1. I simply don't fully understand how background images work on here, given what happens when the site resizes, or shifts to view on mobile: in one iteration last week, I made a rainbow design the background and was constantly frustrated by not anticipating what page sections do in relation to a top section https://thevoterparty.org/home1  It seems like backgrounds on a page aren't a thing? (e.g. this article).  I was admittedly surprised to learn this and I don't know if I get why such a change would be implemented with 7.1. I don't mean to be a whiner I just don't quite get it; and I fear I lack the skills to work around it.