manual.css has a rule like this:
p+h1, ul+h1 {
...
}
This rule means "for headers (h1) tags that come after a paragraph (p) or after an unordered list (ul), do...".
When you add a div between the paragraph and the header, you break that rule. A simple solution is to change the rule like this:
p+h1, ul+h1, h1.myheader {
...
}
and then change the h1 headers in the HTML file to:
...
<h1 class="myheader">
...