Add dark mode support for C++ syntax highlighting

[julioest]

Apply readable syntax colors for cpp-highlight classes when dark
theme is active, consistent with existing dark mode code styling.

Before:

After:

fixes #594

[cppalliance-bot]

An automated preview of the documentation is available at https://596.site-docs.prtest2.cppalliance.org/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2026-02-13 06:44:06 UTC

[mborland]

This is a significant improvement, but the output is still difficult to read since the text is still black. See below:

[julioest]

I'm aware and will fix this soon very soon