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Generally in a Drupal 8 theme, blocks can be placed in regions which are placed in a page.html.twig template. How do you display a block inside a node template?
In your Drupal 8 theme, you can add classes to the body and html classes via your THEMENAME.theme file.
A look at a couple of methods for a div to maintain the aspect ratio of a background image. Also a sass mixin for my preferred approach.
Overriding Drupal core CSS in your theme by modifying core library definitions.