The continue reading "post title" is a tricky little devil, and in the default theme has no extra styling. In fact it is just part of .serendipity_entry_body, however, blogging is deeply personal and every user has different needs.
You should notice that some of the themes on this site don't style the 'continue reading' byline so it appears directly after the content, often breaking the designers hard work.
My belief is that the entries.tpl needs to be amended and the continue reading code needs a <p> placed around it. I also tend to add an additional class to my byline so that I don't have to worry that my paragraphs elsewhere will be affected. Lets assume you add the <p>, your entries.tpl code could look like this,
<p class="continue_reading"><
a href="{$entry.link}#extended">
{$CONST.VIEW_EXTENDED_ENTRY|
@sprintf:$entry.title}</a></p>
Now simply create the following style in your stylesheet,
.continue_reading {
clear:left; }
This has the advantage of also allowing further styling, so you could for example change the color or add an image.