I've managed to customize the header and highlight dropdowns for CKEditor 5 – classic editor build by creating a new custom build.
But I don't know how to add additional buttons to the toolbar. For example a fullscreen button. Current progress: github link
There's no fullscreen button at this moment. You can add +1 to the https://github.com/ckeditor/ckeditor5/issues/1235 to increase its priority or try to implement this feature on your own.
Basically, to create your own button you need to create a plugin which registers the button in the component factory and adds some actions that will be performed on the
buttonView:execute event. You can follow the steps described in creating a simple plugin guide.
A good and complementary reading about the UI library can be found here.
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