fix(shiny-preset): Improve Sass variable support

[gadenbuie]

Fixes #1132

For page_navbar(), don't set CSS vars holding the default background colors if the user gave us $navbar-bg or $navbar-{light,dark}-bg Sass vars.

For page_sidebar(), don't set navbar colors if the user is relying on $navbar-bg.

In the example app, switch between page_navbar() and page_sidebar() to demonstrate that $navbar-bg sets the navbar color as expected (note that inverse is ignored in page_sidebar()).

In page_navbar(), disable the light/dark navbar bg Sass vars to confirm that $navbar-bg wins in both. Then re-enable light/dark bg vars and toggle inverse to see the navbar switch between light/dark variant.

library(shiny)
pkgload::load_all()

# ui <- page_navbar(
ui <- page_sidebar(
  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)