ui.page_navbar() doesn't use the expected theme colors

[gadenbuie]

Possibly related to #882, but more broadly for other themes, including shinyswatch themes. (shinylive example).

from shiny import App, Inputs, Outputs, Session, render, ui
import shinyswatch

app_ui = ui.page_navbar(
    shinyswatch.theme.flatly(),
    ui.nav_spacer(),
    ui.nav("Page 1", "Page 1 content."),
    ui.nav("Page 2", "Page 2 content."),
    ui.nav("Page 3", "Page 3 content."),
    title="Dashboard",
)


def server(input, output, session):
    pass


app = App(app_ui, server)

In R, using the Flatly Bootswatch theme, the navbar is green by default. This may not be the right choice either, but it points to different assumptions that may not be met in py-shiny.

R Code

[Open on shinylive](https://shinylive.io/r/editor/#code=NobwRAdghgtgpmAXGKAHVA6ASmANGAYwHsIAXOMpMAGwEsAjAJykYE8AKAZwAtaJWAlAB0IdJiw71OY4RBEBXWgAIAPAFolqKAHM4AfWgA3ei3YilS0rVLU4SgLxKhYACJQe9IiwAmz3OctuOHgHJSk9UiD4dlRGOE44UlDnADNqKBtWZwF-CAsjPU4tAjhGdhyAgq0IOGozMAAFKBrqJQBGPycwABVeTiVafura9uzc-KhDPWG65yaWpQAmTudewYGh5pHlsAq8pSqt2cajpQBmFZ6+jc1Ti92RWREExkNS1Q0U+QgCKxJ2PioeSkXBKIjAoEgpQJTicWgkARKEAiEQAXxREB4fFYAEF0OxFKCXm9GAIwKiALpAA)

library(shiny)
library(bslib)

ui <- page_navbar(
  title = "Dashboard",
  theme = bs_theme(preset = "flatly"),
  nav_spacer(),
  nav_panel("Panel 1", "This is panel 1"),
  nav_panel("Panel 2", "This is panel 2"),
  nav_panel("Panel 3", "This is panel 3")
)

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

}

shinyApp(ui, server)

This issue was inspired by https://discord.com/channels/1109483223987277844/1183883532855148584