Include CSS & JS
Sometimes, the control panel of your website has different design and layout than the frontend part. For instance, CSS of the main website template defines one set of fonts, and in control panel - another. This may lead to visual discrepancies and displaying problems in the WYSIWYG-editor.
To solve this problem, use Include CSS & JS add-on. On the configuration page in Dashboard specify full URL of those CSS and JS files that need to be loaded on the page that contains the editor.
Please note that if you want to load Bootstrap 4 files, there is no need to specify them manually, simply use the special option in Bootstrap add-on.