What version of Hugo are you using (hugo version)?

hugo v0.118.2-da7983ac4b94d97d776d7c2405040de97e95c03d+extended darwin/amd64 BuildDate=2023-08-31T11:23:51Z VendorInfo=brew

Does this issue reproduce with the latest release?

Yes.

What are the steps to reproduce?

Using the command found in the documentation at https://gohugo.io/content-management/syntax-highlighting/#generate-syntax-highlighter-css

hugo gen chromastyles --style=monokai > syntax.css

This generates a css file where the line highlight is the following.

/* LineHighlight */ .chroma .hl { background-color: #ffffcc }

When you run chroma by itself using the following command

chroma -s monokai --html-styles --style=monokai

it will generate the following css

/* LineHighlight */ .chroma .hl { background-color: #3c3d38 }