Fix OpenAPI viewer for dark mode

What does this MR do and why?

Uses the CSS filter property to invert the text color to make the OpenAPI viewer readable when dark mode is enabled. As SwaggerUI currently does not support dark mode out of the box, this seemed like a simple approach to make it readable again.

Also see: https://github.com/swagger-api/swagger-ui/issues/5327#issuecomment-742375520

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Numbered steps to set up and validate the change are strongly suggested.

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Related to #262641 (closed)