Theme development

I see that Bartik and Classy use this piece of code on page.html.twig.

      <section class="section">
        <a id="main-content" tabindex="-1"></a>
        {{ page.content }}
      </section>

What is the purpose of {{page.content}} and tabindex? It is mandatory to use it when I develop a custom theme?

Thanks.

Hi there - apparently my CSS, and so skills are very rusty. Anyway I am trying to add a logo, with a transparent background, but apparently the Batik theme, the background is inheriting #ffffff, as I understand - but regardless of where I edit in the CSS, the logo pops up with a white background and not the transparent, so the gradient behind is not visible. 

Can anyone show me, where to edit the CSS, or how to make the background transparent, and not white? (preffered before I toss my laptop out of the window)

Thanks

The topic has not been updated for almost a year, when I give rights to edit the nodes, with the barrio theme, the layout looks awful help fix when I used the bootstrap3 theme.
When I go to the node editing page, by the user. How to change a button and its position. I turned on debug, saw the path form-element-label.html.twig, but when it was replaced from bootstrap3 theme, the button remained in "()"

Scenario: I have a custom menu, full of links defined by an end user.

It's inside a custom block, also defined by an end user.

The block has a title the user picked.

I need to have a variable in menu.html.twig that contains the title of the block, as picked by the user.

So, if Sally creates a block named "Avast!" and then places a menu inside it full of links to pirate-themed web sites, I want to be able to put something like {{ block_name }} into menu.html.twig, and have it result in the phrase "Avast!"

I want to display the rendered entity of a group in the TWIG of a node.

In my type of node, I have a field that refers to the rendered entity of the parent group node--article--menu.html.twig :

{{ content.group_content }}

Here is the contents of the TWIG file of the group, the node must render this code without additional markup group--personnel--node-menu.html.twig :

I'm working on a custom Drupal 8.7.8 theme, and have run into an odd issue: the user name in the "My Account" item is missing. The user icon is present, just no text, and it is misaligned.  The source code is as follows:

<a 
	href="/drupal/user"
	title="My account"
	class="toolbar-icon toolbar-icon-user trigger toolbar-item ally-focus-within"
	id="toolbar-item-user"
	>
		<span></span>
</a>

I have removed some ARIA information and some data attributes for legibility.