Tell us what you are trying to do?
I would like to put the picture at the left of the popup; and then next to right of the picture the two other fields.
Or more in general, how do I CSS the popup?
Is there any documentation that you are following?
That editor is essentially an HTML markup editor where shortcodes can be used for dynamic content.
You can add HTML tags (e.g. organise the output in divs), and that can also include style tags, so you can include CSS declarations directly within the same editor, inside style tags.
That should give you the freedom to design the output as you need.