[ruby/rdoc] Modernize RDoc Darkfish template CSS

(ruby/rdoc#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

ruby/rdoc@6cde9edadb

Author: st0012

lib/rdoc/generator/template/darkfish/_sidebar_toggle.rhtml

@@ -1,3 +1,3 @@
-<button id="navigation-toggle" class="navigation-toggle" aria-label="Toggle sidebar" aria-expanded="true" aria-controls="navigation">
-  <span aria-hidden>&#9776;</span>
-</button>
+<div id="navigation-toggle" role="button" tabindex="0" aria-label="Toggle sidebar" aria-expanded="true" aria-controls="navigation">
+  <span aria-hidden="true">&#9776;</span>
+</div>