TOC hidden in dark mode

[alastc]

An ACT spec is currently hiding the table of contents: https://w3c.github.io/wcag-act/NOTE-act-rules-common-aspects.html#contents

(Hovering over one item gives a very feint outline.)

I'm not the author of the spec, but I have dark mode on so noticed. As far as I can tell this is inherited from Bikeshed's styles.

I think that's just an update needed in the .toc a styling.

Also (particularly for an accessibility spec) the text-decoration-color on links should use a lighter colour on a dark background.