Theme development

I'm not getting how to create a sub-theme. I am just hacking Bartik. But I was trying to do it in a child theme. All I want is to add one style.css file so I can make some overrides to the default Bartik theme. Here is my info and libraries file.  I have my style.css saved in the same theme folder as the info and libraries files. 

Thanks for any help. It seems right, but I can't get it to load.

Hello.

New to twig adn drupal.

I am trying to check if a field has value and I use this code

{% if(content.field_info_simeio_synantisis) %}

which is alway true.

Shouldn't this be false from empty fields?

Hello,

I am new to twig and Drupal.

I have an Image Field that can take multiple items.

The following code in twig laways retuns 2 even if there is not image submitted. 

{% for i in 0..node.field_periodic_image_gallery|length-1 %}

Souldn  't this return 0 when I have no image submitted?

Hello,

may be you would have some suggestions examples - how to implement CSS Framework for a Drupal 8 theme...

I have never yet created a new Drupal themes, just been customizing some from Drupal ready submitted themes....

I would like to add to the skin so that the name of the subpage is displayed in a given place, but I do not work?

<div id="name">
	<div class="layout-container">
		<h2>{{ title }}</h2>
		</div>
</div>

I have a base theme. I have a sub-theme. I have a sub-sub-theme.

On sites that have the base theme and sub-theme, the css loads in the correct order: base theme > sub-theme.

On sites that have the base theme, sub-theme, and a sub-sub-theme the css loads in the wrong order: sub-theme > base theme > sub-sub-theme.

I need the order to be: base theme > sub-theme > sub-sub-theme.

All three .libraries.yml files look like this: