Improve code block CSS in libcu++ docs

[Nyrio]

Description

closes #1481

This PR improves the readability of code blocks in the libcu++ docs by:

• Removing a rule that sets the text background instead of the block background.
• Adding a rule for a darker background for code blocks.

Before:

After:

Notes:

• To override some defaults outside the project, I used a more specific selector.
• Local docs building is still broken, see [DOC]: Instructions for hosting docs locally don't work #1309
• I used the color #1a1a1a because I find that #111111 is too high contrast. Most code editors use lower-contrast themes for better readability.

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[miscco]

Looks great to me.

I also like the new background color a lot better than the old one

[miscco]

/ok to test

[ahendriksen]

Thanks! An aesthetic improvement is always welcome :)

[jrhemstad]

Thanks @Nyrio ! This is much better. We've been dragging our feet on fixing this because we're planning on updating the libcudacxx docs to use the same Sphinx template as the CUB docs, but this is a major improvement in the meantime :)

[miscco]

Thanks a lot for making our documentation more readable 🎉