doc: syntax-highlighted JSDoc comments in dark mode are low-visibility

[DerekNonGeneric]

📗 API Reference Docs Problem

• Version: ~16

• Platform: n/a

• Subsystem: docs

Location

Section of the site where the content exists

Affected URL(s):

• https://nodejs.org/dist/latest-v16.x/docs/api/esm.html#esm_loaders

Description

Concise explanation of the problem

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• I would like to work on this issue and
  submit a pull request.

/cc @nodejs/documentation @aduh95