Ensures that the contrast ratio is at least 4.5:1 (so that is complies with WCAG 2.0: https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html).

https://developer.mozilla.org/en-US/docs/Web/CSS/text-size-adjust

CSS is now split into poole.css and hyde.css; chrome/sidebar.html updated.

Also cherry-pick:

  * poole/hyde@48b9d1f - Set a max-width for images