doc: fix color contrast for anchor marks in dark mode

Currently, in its dark mode, our website background color is #090C15 and
our anchor marks are #707070 for a contrast ratio of 3.94 to 1, falling
short of the WCAG AA 4.5 to 1. This changes the mark color in dark mode
only to #7A7A7A for a color contrast of 4.55 to 1.

PR-URL: https://github.com/nodejs/node/pull/39168


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>