Incorporate Improvements from mystjs implementation

[mmcky]

Context

The mystjs team have made some nice improvements to the dark mode in particular

Proposal

Review and incorporate changes:

https://github.com/executablebooks/myst-theme/pull/87

and

https://github.com/executablebooks/myst-theme/pull/90

Tasks and updates

• [ ] @DrDrij to review changes from a design perspective
• [ ] @AakashGfude to setup the changes for sphinx-proof