fix(navbar): Improve navbar colors in dark mode

[gadenbuie]

This rule removed in this PR only changes the background color of the navbar, which is not enough to fully restyle the navbar in dark mode. It might work in the default cases, but it certainly does not work if users have set $navbar-dark-bg or $navbar-light-bg to appropriate values, leading to inaccessible values (e.g. white text on a light background).

This PR removes the changing of the navbar background color in dark mode. This is inline with upstream expectations: Bootstrap expects the navbar colors to be static (i.e. the same in light and dark mode).

If we want to support different navbar colors we'll need to implement something that compliments the current setup.

This PR also sets data-bs-theme="dark" on the navbar <nav> element when inverse = TRUE and fixes the navbar toggle icon in dark mode.

Important

Re-test after merging #1134

Before	After	Description
		Dark mode with inverse = FALSE, black on light blue is expected but white toggle button icon is not.
		Dark mode with inverse = TRUE, white text on dark background is expected in both light/dark, before shows that light navbar bg was used in dark mode.

Example app

library(shiny)
pkgload::load_all()

ui <- page_navbar(
  title = "My Simple App",
  theme = bs_theme(
    preset = "default",
    navbar_bg = "#732400",
    navbar_light_bg = "#b2d8ff",
    navbar_dark_bg = "#00165d",
    cache_buster = format(as.integer(Sys.time()))
  ),
  inverse = TRUE,
  sidebar = sidebar(
    title = "Settings",
    "Just stuff here",
    input_dark_mode(),
  ),
  nav_panel("Home",
    h1("Welcome to the Home Page"),
    p("This is some boilerplate text for the home page."),
    card(
      card_header("Just a regular card"),
      layout_sidebar(
        sidebar = sidebar(
          "Some stuff here too",
          position = "right"
        ),
        "And card content with a sidebar!"
      )
    )
  )
)

server <- function(input, output, session) {

}

shinyApp(ui, server)

[cpsievert]

Is this gonna work sensibly for scenarios where we don't have control over the navbar markup (e.g., pkgdown, rmarkdown, flexdashboard, etc.)?

[gadenbuie]

I can look into this and will add anything that needs to be addressed to posit-dev/brand-yml#52. Bringing pkgdown and bslib back into alignment is part of that plan.

[gadenbuie]

This whole PR does interact with rmarkdown websites and the approach I'd like to take is to make .navbar-default and .navbar-inverse be no-op classes for BS 5+. We could also take them out of the markup when we know that we're in BS 5+. That's going to require another pass at this PR, though.

[cpsievert]

LGTM once the remaining suggestion is addressed. Lets also remember to get the equivalent py-shiny changes in soon after merging