You need to define "para break" in the CSS to get it to do something.
If you want to skip all that, using <p class="chapter"><p> as recommended by deback should do the trick.
This is what I'd do:
Search
Code:
(?ms)(.{400}.*?)(<p[ >])
Replace
Code:
\1<div class="chapter"></div>\2
The \1 and \2 corresponds to the search matches inside the parentheses:
Code:
\1: .{400}.*?
\2: <p[ >]
The above replacement regex basically inserts <div class="chapter"></div> between \1 and \2, and should match any paragraph regardless of class or style. Then convert via Calibre with Structure detection - Chapter mark - pagebreak.