Choose approach for base theme

[flying-sheep]

Description of feature

Concerns:

• Layout: needs to be clean and easy to use, e.g. using simply sphinx toctrees
• Extensions: we need to support MyST notebooks, Sphinx-Design, and maybe more plugins
• Dark mode: has gone through 3 eras, both the theme and plugins need to agree for support (ipywidgets sucks at this). This is the perfect approach: Dark mode like it’s 2024 pydata/pydata-sphinx-theme#2484
• Color system: colors shared with the website, available as CSS variables. see pydata theme docs about this.

Ideas:

• customize PyData theme lightly
• customize PyData theme similarly to book theme but being able to fix issues quickly, e.g. fix: don’t show navbar if the user doesn’t ask for it executablebooks/sphinx-book-theme#987
• other theme base?

Theme	Pro	Con
PyData	Relatively modern CSS makes customization nice: CSS variables, media queries	Looks wonky, header gets big, unclear what belongs into header and what into sidebar[^1]
Book	Looks good and clean out of the box, otherwise see PyData (it customizes it)	Slow response to isses, we’d customize a customization, so more complex overrides
Other	There might be one that’s perfect?	Most don’t have good notebook/MyST support, might use a dark mode scheme not supported by extensions