Add a sticky class to node displayed via View How to add class for sticky nodes in Drupal 8, so I can theme views (sticky having different background etc).

I have seen a post that it can be done through Twig, but I cannot understand in which template should I put the lines and frankly...which lines?
You already get the sticky class on views-row in most themes if your Format Show is set to Content. 

But if it's set to Fields you can do it through the interface with the help of Content: Sticky at top of lists field. 


Add the Sticky at top of lists field from Content Category to your View
Under Output format setting for this field choose Custom and then enter your desired class name into the offered field:
Custom output for TRUE
node--sticky
Custom output for FALSE
[____leave empty____]
Under Rewrite results uncollapse the Replacement patterns and copy the code for this field. It will be something like {{ sticky }}.  

Don't enable Rewriting, this step is just to copy the pattern.  
Save the Field settings and check the views results - do you see the class name listed for nodes that you know are stickied? If yes then edit the field again and Exclude from display
Under Format: Settings paste the pattern into the Row class input field


You will now have the text you entered for the TRUE sticky state appear as one of the classes next to views-row class.