Book theme options to specify theme and disable theme toggle button

[gvarnavi]

Adds the following two options to the book-theme:

site:
  template: book-theme
  options:
    theme: 'dark'
    hide_theme_toggle: true

• theme can either be 'light' or 'dark' and forces the initial theme (as opposed to using local storage)
• hide_theme_toggle is a boolean and disables the top-right theme toggle

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[agoose77]

Hi @gvarnavi,

Thank you for opening this PR!

Theme selectors are an important part of accessibility (though there is nuance to that point). Is there a particular reason that you want to disable this preference for your users of the theme?

[gvarnavi]

Thanks for the prompt reply @agoose77 !

The real use case is that I started modifying the book-theme to achieve a stripped-down "presentation-like" look, similar to what I've been using with Observable Framework.
I was pleasantly surprised to find out I could get most of the way there with site options such as

  template: book-theme
  options:
    logo: ./figures/logo.svg
    logo_dark: ./figures/logo_dark.svg
    hide_outline: true
    hide_title_block: true
    hide_search: true

But wanted more customization to completely style/disable the header, hence the hide_theme_toggle option.

As for locking in the theme, we use a lot of interactive matplotlib figures for the Elemental Microscopy journal, which are challenging to have support both light/dark themes both when static (we should at the very least add a placeholder_dark attribute to the figure directives), but also when dynamic (due to matplotlib not providing transparent backgrounds, and the Jupyter kernel presumably not able to access the current theme/theme changes).

I appreciate the accessibility point, but that requires a level of sophistication I don't think is currently easy to support for interactive figures (in contrast e.g. to MyST tables, which look great in both).

Thanks again for looking into this!