Enable built-in Sass modules

Hi

Is it possible to allow usage of built-in Sass modules? quarto cli discussion #12254 might be helpful.

Here is some starter code for the subsequent examples.

library(bslib)
library(shiny)

custom_theme <- bs_theme(version = 5)

# UI ---------------------------------------------------------------------------
ui <- page_navbar(
  theme = custom_theme,
  nav_panel(
    "One",
    card(
      card_header("card header goes here", class = "bg-aaa"),
      "card content goes here"
    )
  ),
  nav_panel("Two"),
  nav_panel("Three")
)

# Server -----------------------------------------------------------------------
server <- function(input, output) {

}

# Run the application ----------------------------------------------------------
shinyApp(ui = ui, server = server)

In the bslib getting started theming Variables docs we find the example

bs_theme("progress-bar-bg" = "mix(white, orange, 20%)")

We can use the deprecated lighten() function (deprecated as mix() above, but it's less illustrative for mix()) to turn the navbar slightly orange.

custom_theme <- bs_theme(
  version = 5,
  bg = "white",
  "navbar-bg" = "lighten(#e06d33, 30%)",
  fg = "black"
)

How can we @use the Sass color module's scale()?

custom_theme <- bs_theme(
  version = 5,
  bg = "white",
  "navbar-bg" = "color.scale(#e06d33, $lightness: 50%)",
  fg = "black"
)

Similarly, this works

custom_theme <- bs_theme(version = 5) |> 
  bs_add_variables("aaa" = "#e06d33") |> 
  bs_add_rules(
    "
    .bg-aaa {
      background-color: lighten($aaa, 20%);
    }
    "
  )

But we wished to do

custom_theme <-bs_theme(version = 5) |> 
  bs_add_variables("aaa" = "#e06d33") |> 
  bs_add_rules(
    "
    .bg-aaa {
      background-color: color.scale($aaa, 50%);
    }
    "
  )

or

custom_theme <- bs_theme(version = 5) |> 
  bs_add_variables("aaa" = "#e06d33") |> 
  bs_add_rules(
    "
    @use 'sass:color';
    
    .bg-aaa {
      background-color: color.scale($aaa, 50%);
    }
    "
  )

No source file or test is identified in the issue. Start by reviewing the linked Quarto CLI discussion and reproducing the Sass examples; done means built-in modules can be used both in theme variable expressions and in rules added through bs_add_rules().