Rustdoc fails contrast check for a certain element.

[JayanAXHF]

Issue Synopsis

The Since <version> at the start of the docs page has a contrast ratio of 3.1:1 with the background on the dark theme, which fails both AA and AAA specifications for WCAG 2.

The colors for the ayu theme pass WCAG Level AA, but fail Level AAA with a contrast ratio of 4.69:1. The light theme fails both AA and AAA specifications with a contrast ratio of 3.94:1.

Useful Links:

1. WCAG Contrast Guidelines: https://www.w3.org/TR/WCAG22/#contrast-minimum
2. WebAIM Contrast checker: https://webaim.org/resources/contrastchecker/

[arferreira]

@rustbot claim

[arferreira]

Investigating this issue I found that --right-side-color variable is set to grey(#808080) for the dark theme.
Failing AA and AAA contrast ratios as described:

Testing some options:

Passes AA, fails AAA, @JayanAXHF which level should we target AA or AAA?

Also, should I fix light theme and ayu in the same PR? they also use that grey.