By default, Sigil doesn't control the color of the ToC...or any other device/app specific feature. Sigil will create a default sgc-toc.css (and sgc-nav.css for ePub3) but that only includes positioning/structure of the TOC, not color.
Here are the default CSS file(s) created by Sigil.
Spoiler:
sgc-toc.css
Code:
div.sgc-toc-title {
font-size: 2em;
font-weight: bold;
margin-bottom: 1em;
text-align: center;
}
div.sgc-toc-level-1 {
margin-left: 0em;
}
div.sgc-toc-level-2 {
margin-left: 2em;
}
div.sgc-toc-level-3 {
margin-left: 2em;
}
div.sgc-toc-level-4 {
margin-left: 2em;
}
div.sgc-toc-level-5 {
margin-left: 2em;
}
div.sgc-toc-level-6 {
margin-left: 2em;
}
sgc-nav.css
Code:
nav#landmarks {
display:none;
}
nav#page-list {
display:none;
}
ol {
list-style-type: none;
}
You can, of course, create your own css styles manually - OR - add a css file (sgc-toc.css) to your Sigil preferences location. You could include your 'patch' from above in that file if you really want to.
However, I have a feeling that this is caused either by Kobo not honoring the css or, more likely, by some other CSS in your existing ePub. I would look to see if there is some other styling that is interfering. Use the Inspector tool at the bottom of your preview window to see the CSS affecting your TOC...or do a find:color on your css sheets to see where that is found.
Cheers!