Pick up the bundled code-highlighting palettes from site-commons #561

SpineEventEngine/site-commons#35 decided to move the code-highlighting palettes into the theme's SCSS bundle (option B). The change is in SpineEventEngine/site-commons#37.

What changes for this site

Nothing in the templates:

• site/layouts/_partials/head/head.html keeps including theme/head/code-theme.html. The partial now renders only the inline theme script, with no <link id="code-theme-css">.
• site/assets/scss/main.scss already imports theme-main, so the palettes come with main.css.

After the update:

• Pages no longer request /code-theme/dark.css and /code-theme/light.css. These files are no longer published.
• <html data-theme> is set before the first paint on every page, including first visits. Until now it was set on DOM ready. site/static/libs/docsearch/variables.css keys off html[data-theme='dark'], but its dark values are the same as the light ones, so the search modal does not change.

To do

Once SpineEventEngine/site-commons#37 is merged:

• Update the theme in site/ with hugo mod get -u github.com/SpineEventEngine/site-commons, then commit go.mod and go.sum.
  The current theme already requires Hugo v0.169.0, and CI uses v0.161.1. So this update goes with the Hugo upgrade (Upgrade Hugo to v0.169.0 documentation#89).
• On a docs page with code blocks, check that:
  • the toggle above a code block switches the colors at once;
  • after reloading with the light theme selected, no dark token colors show while the page loads;
  • in a private window, code blocks are dark and have their background from the first paint.