Block Style Plugins is an API module that allows a module or theme to add style options to block configuration by creating a custom plugin. It is advisable to do this primarily in themes since the majority of styles should be specific to a theme.
It allows you to define form fields on a block's configuration page which will apply classes to that block. These styles can also be restricted to only show on certain blocks so that editors aren't confused by options which shouldn't be available to them.
How to setup and use the Drupal 8 Block Style Plugins module
Learn how to create an advanced plugin with PHP.
Restrict the visibility of styles to specific blocks with include/exclude.
Find some help by looking at example plugins
Apply a twig template suggestion for a block
This is a fancy way to allow editors to add classes to blocks via the UI.
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