Review the sphinx-book-theme for a11y issues that are too large to deal with

[feanil]

If we find major issues, we should work to fix them upstream or consider moving to a more accessible theme with a strong preference for fixing up-stream over maintaining our own theme.

[feanil]

There weren't too many issues but one that was on the main page was a contrast issue with the "Home" buttons.

I tried to override the primary color for the design theme to deal with this but there seems to be a bug where our custom.css file loads earlier than the design-style.css file that the extension provides.

I made executablebooks/sphinx-design#124 upstream to just fix the upstream theme and I'm gonna spend a little bit more time to see if I can figure out why the CSS is loading in the wrong order.

[feanil]

#265 is a work-around for now until we can land the upstream PR and re-visit this.

After this PR lands, I think we can call this done for now.

[feanil]

PR has landed, so this should be all set.