AFAIK, it's not possible to style NCX entries. The only thing you can do is manually insert bullet characters before the chapter headings or its title attributes. For example:
Code:
<h1 title="◯ Chapter One">Chapter One: The Beginning</h1>
Sigil will use whatever text you define in the
title attribute to generate the NCX and HTML TOC headings.
BTW, any changes that you implement in the HTML TOC will not be automatically implemented in the NCX TOC.