Theme development

Hello! I'm totally fresh with drupal and I'm currently working on 9.1.5 with bootstrap3 theme that contains child subtheme. Our main site is full of custom blocks, modules, script injectors and etc. I'm planning to develop and release it on drupal 10.x.x, but I've no clue how to export all those settings and prepare for next major update. What should I first do?

I'm looking to change the colors of these boxes, and could use some help if anyone is willing. 

https://designsystem.digital.gov/components/accordion/

I'm not sure where to put stylings, and I'm struggling with the documentation.

Hi! Can you help me? How to insert Views Block to node (auto) without modules.

I have Content Type - Articles  I create tempalte page--article.html.twig and paste code from page.html.twig

My Views Machine name random_article block_1

What should i do next?

Using Menu Item Extras module, it is possible to add new fields to the menu items.

But when dealing with the Olivero theme, newly added fields are not shown.

The invocation method of the olivero.theme:

I'm building a service that aggregates and/or syndicates news. I'm open to theme recommendations. The features needed: compatibility with the Feeds module, the ability to search by location and/or topic, easy creation of either a full news story or a short brief. The theme would need to be clean and easily navigated by users.

I've done some checking and there a several directory themes that partially meet those requirements, but they are largely based on providing business overviews.

In a form created by the cli, fields are displayed with the label above and the input below.
I need to display, in this manner, multiple fields horizontally
I have done various tests with flex

...['#prefix'] = '<div class="flex-container-row">' ;

but I did not succeed
The CSS is this
 

.flex-container-row {
  display: flex;
  flex-direction: row;
  margin-top:20px;
 }