Consider updating Book theme

[pryrt]

While thinking on #653, I was reminded that at one point, someone had requested the User Manual switch to Dark Mode, which I wasn't going to do, because there are still plenty of people who prefer light mode. But #653 reminded me about that, and I started searching for whether there's a way to have a hugo-based site automatically or manually switch from Dark to Light (auto = depends on browser settings).

The conclusion of https://github.com/alex-shpak/hugo-book/issues/151 does indicate that the feature has been added as the parameter BookTheme: auto|dark|light . I will have to experiment to see if that really works, and whether I can import the updated book theme without affecting our customizations (as far as I can tell, only _custom.scss has been updated since we started 2019... but I don't know what else might have been customized before the first commit)

If I get that to work, I will want to make sure anything from #653 and from my earlier <KBD>-styling, will look right under both light and dark.

[pryrt]

Notes to self:

• hugo-book v9 requires hugo68.3 or newer

• config.toml add:

  [params]
    # (Optional, default light) Sets color theme: light, dark or auto.
    # Theme 'auto' switches between dark and light modes based on browser/os preferences
    BookTheme = 'auto'
  
  [markup.goldmark.renderer]
  unsafe= true

  • params for setting auto mode
  • renderer to allow "unsafe" raw html (since hugo 0.60)
• to use themed variables in the <KBD> dropshadow, use the var(...) syntax, and the variable names defined in the @mixins sections of ...\assets\_defaults.scss:

  /* drop shadow */
  -moz-box-shadow: 1px 1px 10px 1px var(--gray-500);
  -webkit-box-shadow: 1px 1px 10px 1px var(--gray-500);
  box-shadow: 1px 1px 10px 1px var(--gray-500);

• the only way I can figure out how to add my own variable to either of the @mixins is to edit ...\assets\_defaults.scss : I was hoping to avoid editing anything but _custom.scss, but might have to stick with that...