pimcore_editmode_wysiwyg.js

/* /website/static/js/editmode/wysiwyg.js */
CKEDITOR.stylesSet.add( 'custom_styles', [
      // Paragraph Styles
      { name: 'Regular Text', element: 'p', attributes: { 'class': '' }},
      { name: 'Large Text', element: 'p', attributes: { 'class': 'content__text-L' } }

]);

CKEDITOR.editorConfig = function( config ) {
    // add our custom styles
    config.stylesSet = 'custom_styles';
    config.toolbar = 'Basic';
};
/* use in Pimcore like this:
  <?= $this->wysiwyg('custom_wysiwyg', ["customConfig" => "/website/static/js/editmode/wysiwyg.js"]); ?>
*/