footer: simplify coloring
Instead of setting the color everywhere just to avoid setting the color on the headers, set the colors on the headers, and make links in the footer inherit the color setting. While we're at it, set the color to text-dark instead of text-body (which is the default). This just decreases the contrast here ever so slightly, to give these heading a tiny bit less emphasis. The difference is almost impossible to see, but it makes a bit more semantic sense.