I don't know if I have the solution but the case is similar to the
one I had with the ToC, which looked light grey and I wanted it to be black. I found a solution but I don't know enough to understand it. Instead of applying ‘
colour:black’, using ‘
-webkit-text-fill-color:inherit’ the ToC text looks black (I use epub edited with Sigil and convert it to kepub with Calibre).