Classy blocks is a WordPress plugin that makes adding additional CSS classes a breeze in the new WordPress blocks editor (gutenberg). Instead of typing in a CSS class in the Advanced -> Additional CSS Classes textbox, let your editors select a CSS class in a dropdown you create.
Create a block field that appears in the Inspector with a user-friendly UI. Add your CSS classes through the WordPress Customizer’s Additional CSS or your theme’s stylesheets. Then blocks can have custom css classes with the click of a dropdown just like this block. This block has the bottom green border and padding added with the plugin.
Download the Classy Blocks plugin in the WordPress plugin repository: http://wordpress.org/plugins/classy-blocks