🌙 Add Dark Mode Support to Mediumish Theme

[Nicco-Cirone]

This pull request introduces full dark mode support for the Mediumish Jekyll theme. The implementation automatically applies dark styling based on the user's system appearance settings (prefers-color-scheme: dark).

✅ Key Changes:

• Added dark.css for scoped dark mode styling:

1. Applies to layout, typography, navbar, cards, footers, images, forms, and buttons.
2. Includes styles for the .rating-holder star rating block and the .alertbar newsletter signup.

• Introduced a jumbotron_dark.webp background for consistency
• Removed inline styles conflicting with dark mode overrides
• Ensured fallback to light mode remains fully functional

🔧 Compatibility Note:
To ensure successful GitHub Pages deployment via Actions, the following line was added to the Gemfile:
`gem "jekyll", "~> 4.3"
This resolves compatibility issues with older dependencies such as ruby_dep.

🙌 Author & Usage:
This dark theme was created by Nicco Cirone and is currently in use on niccocirone.com/blog.

I hope it can be a useful enhancement for others using the Mediumish theme.
Happy to adjust or refine based on any feedback!