New [Preview] in Flow is great (card 348, bug 57158).
It doesn't reflect how a header area element or topic title will appear. Header text will appear medium gray #777, and topic text will be big and on a gray background, but all items appear in the same light-colored, yellow-bordered 'flow-content-preview' box. Topic text is also confusing because one preview button previews both it and your first post (cool), but they appear in a single preview box.

IMO the preview should not look identical to final display since it will look like another header/topic/post, but it should resemble the final appearance. Maybe keep the border and background, but allow the font CSS to apply to header and topic title; put the topic title and first post in separate boxes and make the former slightly gray.

Version: master
Severity: normal