[DOCS] Request for Dark Mode on docs site

**Issue Type**
- [ ] Existing documentation (e.g. typos, errors, outdated information)
- [X] Suggest new documentation that is currently missing

**Current Behavior or Documentation Gaps**
For existing documentation:
FastHtml has been very fun so I end up working on it into the night. Unfortunately when referring to docs I am blinded regularly due to a lack of dark mode or detection on the site.

For new documentation suggestions:
Implement dark mode on site.

**Relevant Links**
Dark mode by example https://www.fastht.ml/docs/tutorials/by_example.html#styling-basics

**Confirmation**
Please confirm the following:
- [X] I have checked the existing issues and pull requests to ensure this documentation issue hasn't been reported before.
- [X] I have read the project's documentation and am confident this issue or suggestion is valid.