Navbar bg color

[cpsievert]

Closes #251 and #220

Adds new $navbar-light-bg and $navbar-dark-bg BS4 Sass variables for setting the background color on .navbar-light/.navbar-default (i.e., navbarPage(inverse = FALSE)) and .navbar-dark/.navbar-inverse (i.e., navbarPage(inverse = TRUE)). For example,

library(shiny)

ui <- navbarPage(
  title = "My site",
  theme = bslib::bs_theme("navbar-light-bg" = "lightblue"),
  tabPanel("A", "a"),
  tabPanel("B", "b")
)

shinyApp(ui, function(input, output) {})

And

library(shiny)

ui <- navbarPage(
  title = "My site", inverse = TRUE,
  theme = bslib::bs_theme("navbar-dark-bg" = "darkblue"),
  tabPanel("A", "a"),
  tabPanel("B", "b")
)

shinyApp(ui, function(input, output) {})