How to add css class on click of CKeditor table button?
You can add copies of:
ckeditor.styles.js
ckeditor.config.js
to your theme directory

You can add a class using jQuery addClass

Be sure to set custom JS path under directory CKEDITOR profile settings.

*This was method under D7

D8
Option 1: You use this editor_js_settings hook to adjust ckeditor settings. A full list of settings can be found [here][3]

Option 2: Target item using jQuery addClass in the themes or custom module sites scripts.js file

Option 3: Create a custom module based on a copy of CKEDITOR module with a class added where you need it, or create a patch for the module to add your class. Classes to buttons are added here core/modules/ckeditor/ckeditor.admin.inc (be sure not to edit this core file)