fix(preset): Follow $bslib-sidebar-bg when in dashboard mode

[gadenbuie]

Fixes #1126
Also allows $bslib-sidebar-bg to set $bslib-sidebar-fg with contrasting color if set by the user.

For testing: comment out the bslib-sidebar-bg lines to switch between setting background color values, same for fg colors, toggle preset = "default" to switch betwen presets.

library(shiny)
pkgload::load_all()

ui <- page_navbar(
  title = "My Simple App",
  theme = bs_theme(
    # preset = "default",
    "bslib-sidebar-bg" = "#732400",
    "bslib-sidebar-bg" = "#fcbb9c",
    # "bslib-sidebar-fg" = "#732400",
    # "bslib-sidebar-fg" = "#fcbb9c",
    cache_buster = format(as.integer(Sys.time()))
  ),
  sidebar = sidebar(
    title = "Settings",
    "Just stuff here"
  ),
  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)