Upgrade docsy theme to make use of the dark mode

See https://main--docsydocs.netlify.app/docs/content/lookandfeel/

Upgrading the docsy theme may be non-trivial as we have a number of local customisations (e.g. css overrides to tailor the look to our needs) that may not work with the latest version of the theme.

Start with the linked Docsy look-and-feel documentation and inspect the repository's local CSS overrides and other theme customisations. Compare those customisations with the upgraded theme's dark-mode support; done means the documentation site uses dark mode without breaking its tailored appearance.

Written by the indexing model from the issue text.