...

var synapEditorConfig = {
	...
	/**
	 * Set style url to preview.
     */
	'editor.preview.style.urls': [
		'horizontalLineExtension.min.css의 url'
	]
	...
};

Setting style URL when using editor iframe mode

RELEASE 2.10.0 OR ABOVE

When the editor is used in Iframe mode, it is displayed in the default style unless a separate style URL is set.

For more information on iframe mode, refer to [Installation and Settings> Environment Settings > Basic UI Settings > iframe mode].

Reference

On the screen where the plugin's CSS is not set, the horizontal line is displayed as the default style.

...