background bug with zenburn in gitbook output format

Hello,

I think this is a bug that was addressed in in rmarkdown issues #1471; however, the bug seems to be persistent when rendering a git book format. Essentially, the background color for the code blocks in the gitbook render are not the correct color.

output:
  bookdown::gitbook:
    highlight: zenburn

Session Info

> xfun::session_info('bookdown')
R version 3.6.1 (2019-07-05)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 18.04.3 LTS, RStudio 1.2.1335

Locale:
  LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_US.UTF-8       
  LC_COLLATE=en_US.UTF-8     LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
  LC_PAPER=en_US.UTF-8       LC_NAME=C                  LC_ADDRESS=C              
  LC_TELEPHONE=C             LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

Package version:
  base64enc_0.1.3 bookdown_0.12   digest_0.6.20   evaluate_0.14   glue_1.3.1     
  graphics_3.6.1  grDevices_3.6.1 highr_0.8       htmltools_0.3.6 jsonlite_1.6   
  knitr_1.24      magrittr_1.5    markdown_1.1    methods_3.6.1   mime_0.7       
  Rcpp_1.0.2      rmarkdown_1.14  stats_3.6.1     stringi_1.4.3   stringr_1.4.0  
  tinytex_0.15    tools_3.6.1     utils_3.6.1     xfun_0.8        yaml_2.2.0

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rstudio / bookdown

background bug with zenburn in gitbook output format #761