Dark mode documentation

### Description of the feature request:

Consider https://bazel.build/rules/testing for instance - The code snippets have a dark mode toggle. I would like the same to apply throughout the page.

### What underlying problem are you trying to solve with this feature?

Using bazel docs in dark mode

### Which operating system are you running Bazel on?

Linux

### What is the output of `bazel info release`?

release 5.1.0

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse master; git rev-parse HEAD` ?

_No response_

### Have you found anything relevant by searching the web?

No, unfortunately

### Any other information, logs, or outputs that you want to share?

_No response_