Problem with visibility of .Rmd files

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I tried to publish a post as `.Rmd` file with a YAML front matter. Everything works fine until a header appears (e.g. ## My Title). Header and all the text thereafter appears with an extreme light gray, so that one even almost can't read the text.

But there is no problem with the exact same file using `.Rmarkdown` file ending instead of a `.Rmd` ending. `.Rmarkdown` is rendered by **Blackfriday** whereas `.Rmd` is rendered by **Pandoc**. There are slight differences, but there shouldn't be any different with standard Markdown. I am using blogdown but I tested `.Rmd`- files with other Hugo themes and had no problems.

I tried it also with a fresh installation out of the box so that I could prevent any issue of some changes I already had made to the template. So I do not have a clue. Probably it has to do with a conflict with the CSS resulting after Pandoc has rendered the file.

Do you have any idea what to do?

Compare the rendered output of the `.Rmd` and `.Rmarkdown` examples, then inspect the theme CSS affecting headers and subsequent text. Confirm the cause by reproducing the contrast problem in a fresh Hugo theme installation; done means `.Rmd` headings and following text remain readable.

Written by the indexing model from the issue text.