TOC text is light gray (at least on Kobo), and an unexplained workaround

[DiapDealer]

Also note that in my test, the "user agent stylesheet" section shows the following:

a:-webkit-any-link {                                     user agent stylesheet
    color: -webkit-link;
    cursor: pointer;
    text-decoration: underline;
}

The line through the color attribute indicates that it's been applied, but overridden. And as already mentioned, the user agent stylesheet is not really all that relevant to the discussion. It does not "travel" with the epub. It's built into the software/firmware that's used to render the epub (and changes from device to device, app-to-app, browser to browser).

Follow Turtle91's advice for "patching" your epubs to look the way you want them to on your Kobo (or check out the Kobo section of the forum for finding out how to customize your Kobo's firmware).