Theme development

Hi,

Hello!

I'm trying to build my own theme from scratch to have only what I need for my sites.
I have less regions than the system's own, and I get an error :

Notice : Undefined index: sidebar_first dans include() (ligne 138 dans /.../modules/system/page.tpl.php).
The "sidebar_first" region isn't used in my theme, causing an error.

The regions I'm using are declared in the .info file and show up correctly in the blocks page, but aparently my page.tpl.php doesn't fully override the one from the system. Any idea what I should do ?

Thank you !

I have created block using html.twig,made enabled in structure/block layout and it appeared in region where i placed. So, the confusion is it shows only title of the block in that region. I have placed an image which is not shown there,
Do i need to exaggerate the blocks height and width. ?
How can i increase the rows and columns of the block?
Do i want to make something in theme files , regarding block ?

Can anyone explain clearly.

Here is my code of html.twig file

Is there a way I can output the link of an entity reference aside from displaying the label. I have an entity reference field that links to a page node content type. I'm trying to figure out how I can output the link.

Here is the sample of the code I've just created. It shows the label of the entity but it does not come with link.

I tried using `entity.uri.value` or just `entity.uri` to output the url

Hi:
I am going mad here, my default theme is zircon and when I view the page it is using bartik theme's logo.svg file, totally crazy. I am not sure how that is happening and why.

Under page.html.twig

 img src="/core/themes/bartik/logo.svg" alt="Home" 

How could this happen ?
Do I have to replace that logo.svg in bartik with my logo ?

Hello, :)

If there is perhaps Tutorial videos or instructions, around the design for a well-chosen Theme
to adapt or has somebody collected there already experiences?

In general to the design / construction of contents I have a few questions: