How do I set syntax highlighting style in API Use

Hey everyone,

quick question: How do I set a custom syntax hightlighting style, when using nbformat via API. Configuring it that way has no effect:
```py

c = Config()
c.CSSHTMLHeaderPreprocessor.style = "monokai" # Should set different style
html_exporter = HTMLExporter(config=c)
(body, _) = html_exporter.from_notebook_node( nbformat.reads(content))
```
I couldn't find something in the Documentation [only for CLI use](https://nbconvert.readthedocs.io/en/latest/highlighting.html).

Thanks in Advance!

Start with the API example using Config, CSSHTMLHeaderPreprocessor, HTMLExporter, and nbformat.reads, then compare it with the linked nbconvert highlighting documentation. Clarify whether custom syntax highlighting can be configured through the API and document the supported usage and expected result.

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