Theme development

So I am converting one of my D7 sites to D8 and have decided to go all in on using the new layout builder. I think it's really awesome and will be great for our content editors. I'm wondering if anyone is aware of any themes on the marketplace anywhere that already come with some layout builder layouts perhaps using Bootstrap or even better some custom block types and TWIG templates that I could then embed predefined widgets already styled in the themes style into layout builder.

Have fresh Drupal 8 install with Bootstrap 3 and sub-theme created as per: https://www.webwash.net/getting-started-bootstrap-drupal-8/

Have a couple articles listed in the homepage in <div class="views-row">. As per above tutorial, I'm taking variable from bootstrap/assets/stylesheets/bootstrap/_variables.scss and customizing them in scss/_default-variables.scss

Just trying to put a border on this class but having a hard time figuring out where exactly to do it.

I'm struggling to override the template on the admin interface for a content type. I'm going to need to do several modifications to the edit form to meet the editor's requirements, however, I seem to be unable to get this to override.

The page source gives me the following

http://gruplaela.uji.es/__D8/node/39

On this page I have a field called "Authored by" and the field type is Entity Reference. I'm trying to style it but I cant get it to sit inline like the other fields (which are field type Text). Rather the two field items sit on top of each other and push the following field to the right (as if it were float: left in CSS).

What I would like to have is something like this:

Authored by: Patricia Salazar Victòria Codina

In Drupal 7 I could create an external .php file and call it from inside a block by including the php file  

<?php
include_once("./".$theme_path ."/blocks/header_bar.php");
?

Is it possible to call an external twig file in Drupal 8 from a block? 

Thanks,

Nick

hi to everyone. I'm very new. I'm german and apologies my english :/.