Need to be able to set navbar colors from Sass

[gadenbuie]

Currently, in the Shiny preset, setting $navbar-bg does not affect the navbar in page_sidebar() and might affect the foreground – but not background – color of the navbar in page_navbar().

shinylive app

With brand.yml, setting the navbar background color is a common customization and the most natural solution that would work across Quarto and Shiny would be to use defaults.bootstrap, e.g.

defaults:
  bootstrap:
    defaults:
      navbar-bg: $brand-purple

This, however, does not work in Shiny for the Shiny preset because of dashboard-style customizations that we use.