(https://github.com/ruby/rdoc/pull/1157)

- Update color scheme with muted green tones and improved contrast
- Enhance readability by adjusting font sizes and weights
- Improve code block styling with a light gray background
- Refactor layout for better responsiveness and sidebar presentation
- Standardize link styles across the document
- Implement CSS variables for easier theme customization
- Adjust heading styles for better visual hierarchy
- Enhance table and list styling for improved readability
- Optimize search field and navigation toggle appearance
- Improve method detail and documentation section styling
- Reorganize css and overhaul the sidebar design
- Improve code block's syntax highlighting

https://github.com/ruby/rdoc/commit/6cde9edadb