Clarify theming of dropdown in dark mode

[xx02al]

For some $dropdown- variables, {bslib} has light and dark mode variants.

https://github.com/rstudio/bslib/blob/bc56d8e7671b6c1c41c041b6992e24cf1a4bc05f/inst/lib/bs5/scss/_variables.scss#L1296

When building a pkgdown site with light-switch: true, I expect the navbar dropdowns to respect the dropdown-dark-* variables defined in _pkgdown.yml. Instead, these variables appear to be ignored—dropdowns remain in light mode (no [data-bs-theme="dark"] applied) unless I add custom SCSS rules.

So far, I haven't understood why these variables are being ignored. I would appreciate understanding this and whether this is intended behavior or if we can change it.

The _pkgdown.yml and extra.scss files below can be used to illustrate the issue (colors may be sufficient for the actual purpose but not for real life). Build the site with and without extra.scss.

_pkgdown.yml:

url: ~
template:
  light-switch: true
  bslib:
    version: 5
    preset: default
    # Make it obvious which mode we are in
    body-bg: "#F0ECDA"
    body-bg-dark: "#192D3C"
    body-color: "#152740"
    body-color-dark: "#F0F0F0"
    navbar-light-bg: "#487F86"
    navbar-dark-bg: "#96AAAA"

    # What we care about in this issue
    dropdown-link-hover-bg: "#797292"
    dropdown-dark-link-hover-bg: "#23857A"
    dropdown-link-active-bg: "rgba($dropdown-link-hover-bg, .4)"
    dropdown-dark-link-active-bg: "rgba($dropdown-dark-link-hover-bg, .4)"
    dropdown-link-active-color: "$body-color"
    dropdown-dark-link-active-color: "$body-color-dark"

extra.scss

/*-- scss:rules --*/

[data-bs-theme="dark"] .dropdown-item {
  color: $body-color-dark;

  &.active, &:active {
    background-color: rgba(#C88200,.4);
  }

  &:hover, &:focus {
    background-color: #C88200;
  }
}