Post #3240 header and the preview margin

Hello fox! Does anyone know how to hide the header Post #3240 that is automatically generated on my site? I’m using the free version Beaver Builder. The plugin is asking for a css-selector to do it. fl-post-header in global settings doesn’t help.
In the preview, a margin of three centimeters is left on both sides, even if the default row contents are set to the full screen width. Pages I have created with other editors display correctly.