This isn’t something you would do with formatting in the editor—it’s really better to think of the contents link list as just that, a list of links, one per line, that will be used to construct the internal navigation structure of the ebook—because that is much more like what they will end up as in the book, as opposed to some kind of formatted page.
Here is an example what that is going to look like in the contents.xhtml file (indented for clarity):
<nav epub:type="toc" id="toc">
<h1 class="heading-1" id="doc2">Contents</h1>
<ol>
<li><a href="body1.xhtml">Chapter One - Red Book</a></li>
<li><a href="body1.xhtml#doc4">scene a</a></li>
<li><a href="body1.xhtml#doc5">scene b</a></li>
<li><a href="body2.xhtml">Chapter Two - Black Book</a></li>
<li><a href="body2.xhtml#doc7">scene c</a></li>
<li><a href="body2.xhtml#doc8">scene d</a></li>
</ol>
</nav>
You could, if this were the very end of the editing process, copy and paste the result out of the source and into Scrivener, setting the block to a style named “Raw HTML Block” (we have a built-in style for the Ebook compile format that will, when used, pass what you type in straight through to the output). Naturally if you so much as add a preface or shuffle a chapter, it’ll have to be regenerated though, since the internal files are dynamically generated and numbered.
But, with that approach, you could do something like this in the editor:
<nav epub:type="toc" id="toc">
<h1 class="heading-1" id="doc2">Contents</h1>
<ol>
<li><a href="body1.xhtml">Chapter One - Red Book</a></li>
<li><a href="body1.xhtml#doc4">scene a</a></li>
<li><a href="body1.xhtml#doc5">scene b</a></li>
<li class="toc-separator"><a href="body2.xhtml">Chapter Two - Black Book</a></li>
<li><a href="body2.xhtml#doc7">scene c</a></li>
<li><a href="body2.xhtml#doc8">scene d</a></li>
</ol>
</nav>
So now you have a classed <li> element in there that you can visually accentuate with CSS. Dropping something like the following into your compile format’s CSS pane would add a single line of space above that line:
.toc-separator { margin-top: 1em; }
This technique has a lot of flexibility, naturally. You could indent hierarchical entries, change font sizes, colours, etc. For best results, stick with using classes on the given elements though, the rules for what go into the element are fairly strict, but ebook readers won’t care how you modify the visual presentation of the list.
Attached below you will find a working demonstration of the concept to play with.