sass warning when using flexdashboard with with bslib theming

I’m not sure this warning is intended in this case.

---
title: "Fastfood"
output: 
  flexdashboard::flex_dashboard:
    theme:
      primary: "#003366"
runtime: shiny_prerendered
---

BOB
=====================================

Burger

library(flexdashboard)
writeLines(c(
"---",
"title: Fastfood",
"output: ",
"  flexdashboard::flex_dashboard:",
"    theme:",
"      primary: #003366",
"runtime: shiny_prerendered",
"---",
"",
"BOB",
"=====================================",
"",
"Burger"), "sample.Rmd")
options(warn = 2)
rmarkdown::render("sample.Rmd")

Error: (converted from warning) Character vector names are ignored. Instead of a named character vector, use a named list to define Sass variables.

> xfun::session_info('flexdashboard')
R version 4.0.4 (2021-02-15)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 19041), RStudio 1.4.1103

Locale:
  LC_COLLATE=English_United States.1252 
  LC_CTYPE=English_United States.1252   
  LC_MONETARY=English_United States.1252
  LC_NUMERIC=C                          
  LC_TIME=English_United States.1252    
system code page: 65001

Package version:
  base64enc_0.1.3          BH_1.75.0.0              bslib_0.2.4.9002        
  cachem_1.0.4             colorspace_2.0.0         commonmark_1.7          
  crayon_1.4.1             digest_0.6.27            ellipsis_0.3.1          
  evaluate_0.14            farver_2.1.0             fastmap_1.1.0           
  flexdashboard_0.5.2.9000 fs_1.5.0                 glue_1.4.2              
  graphics_4.0.4           grDevices_4.0.4          highr_0.8               
  htmltools_0.5.1.1        htmlwidgets_1.5.3        httpuv_1.5.5            
  jquerylib_0.1.3          jsonlite_1.7.2           knitr_1.31              
  labeling_0.4.2           later_1.1.0.1            lifecycle_1.0.0         
  magrittr_2.0.1           markdown_1.1             methods_4.0.4           
  mime_0.10                munsell_0.5.0            promises_1.2.0.1        
  R6_2.5.0                 rappdirs_0.3.3           RColorBrewer_1.1.2      
  Rcpp_1.0.6               rlang_0.4.10             rmarkdown_2.7.4         
  sass_0.3.1.9001          scales_1.1.1             shiny_1.6.0             
  sourcetools_0.1.7        stats_4.0.4              stringi_1.5.3           
  stringr_1.4.0            tinytex_0.30             tools_4.0.4             
  utils_4.0.4              viridisLite_0.3.0        withr_2.4.1             
  xfun_0.22                xtable_1.8.4             yaml_2.2.1

rstudio / flexdashboard

sass warning when using flexdashboard with with bslib theming #320