accessibility testing of dark mode

[ErinBecker]

As noted in #124, there is need from some additional accessibility testing of the new dark mode feature. Testing will include running rendered lesson pages through WAVE Web Accessibility Evaluation Tool, particularly to test for appropriate contrast. Tested lessons should include those with all possible lesson life-cycles (pre-alpha, alpha, beta, stable) and callout boxes (use https://carpentries.github.io/sandpaper-docs/component-guide.html, as it shows all possible callout boxes).

[froggleston]

The main PR for dark mode has now been merged, so we can move forward with testing accessibility for our official lessons.