[Doc] Fix busted AI assistant CSS

[peytondmurray]

Why are these changes needed?

The docs upgrade (#41115) broke light styles for the ray AI assistant. This PR fixes the styles to make the assistant usable again.

There are a few changes which would be nice to make but are more invasive, and therefore out of scope for this PR (such as centering the placeholder text in the textarea element, which really isn't possible unless you use an input or div instead; see https://github.com/ray-project/ray/blob/master/doc/source/ray-overview/examples.rst?plain=1 for an example of how to do this).

Light mode:

Dark mode:

Related issue number

Closes #41440.

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[angelinalg]

Thank you for turning this around so quickly, @peytondmurray.