Fix accessibility

[ru-fu]

The accessibility check returns some failures on the style guide - which is probably a good benchmark, since it contains most of the formatting we use.

Check the action and fix the failures (might just be one issue?).

[tonyandrewmeyer]

I'd love to see this addressed - we're actively using the theme for the ops API reference docs (and updating to the latest version of the theme at the moment).

In particular:

• The blue (#06c) text on the black (#131416) background has a ratio of 3.31:1 (e.g. with WebAIM's checker or many other similar tools). This fails against WCAG (apart from large text AA), and really is legitimately difficult to read in normal-sized and regular weighted text.
• The grey bar (color-api-background) in dark mode would be great to change anyway, but right now the background (#cdcdcd) with the white (#ffffffcc) text is 1.58:1 contrast :disappointed:.

I haven't looked through the list in the action output in detail but they seem to be mostly dealing with colour contrast, so I imagine the above are included there.