Not sure this is pertinent, but this still works on my Win10 machine using Calibre 6.1
The file ...
C:\Users\<used>\AppData\Roaming\calibre\resources\ templates\inline_toc_styles.css
... has the CSS that the inline TOC uses
Code:
<title>Table of Contents</title>
<style type="text/css"><![CDATA[
/* >>>> inline_toc_styles.css inserted here <<<< */
]]></style>
</head>
<body id="calibre_generated_inline_toc">
I have the CSS that makes the inline TOC formats just the way I like 'em