TheCubicWebBook #890382 fix html colors [open]

the current sphinx stylesheet for the doc has a few issues:

  • the background color for code sections used to be green and is now grey, but green was better
  • the purple color for the Note and Warning sections does not look good with the main orange color
  • the breadcrumbs are framed with brown lines... but I am not sure if it is the color or the top-padding that makes it look weird.
  • we should check for accessibility (contrast, size, etc)
done in<not specified>
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