Modernize RDoc Darkfish template CSS

- 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

These changes aim to create a more modern, readable, and visually appealing
documentation template while maintaining the core Darkfish design.

lib/rdoc/generator/template/darkfish/css/rdoc.css

@@ -101,16 +101,45 @@ h6:target {
 code,
 pre {
   font-family: 'Source Code Pro', Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace;
-  background-color: #f1f3f5;
-  padding: 0.2em 0.4em;
-  border-radius: 4px;
-  font-size: 0.9em;
+  background-color: #f0f4f8;  /* Slightly darker gray background */
+  border: 1px solid #d1d5da;
+  border-radius: 6px;
+  padding: 16px;
+  overflow-x: auto;
+  font-size: 14px;
+  line-height: 1.45;
+  margin: 1em 0;  /* Add some vertical margin */
 }
 
 pre {
-  padding: 1em;
+  background-color: #f0f4f8;  /* Slightly darker gray background */
+  border: 1px solid #d1d5da;
+  border-radius: 6px;
+  padding: 16px;
   overflow-x: auto;
-  border: 1px solid var(--border-color);
+  font-family: 'Source Code Pro', Consolas, Monaco, 'Andale Mono', 'Ubuntu Mono', monospace;
+  font-size: 14px;
+  line-height: 1.45;
+  margin: 1em 0;  /* Add some vertical margin */
+}
+
+/* Syntax highlighting */
+.ruby-constant   { color: #005cc5; }  /* Blue */
+.ruby-keyword    { color: #d73a49; }  /* Red */
+.ruby-ivar       { color: #e36209; }  /* Orange */
+.ruby-operator   { color: #005cc5; }  /* Blue */
+.ruby-identifier { color: #6f42c1; }  /* Purple */
+.ruby-node       { color: #22863a; }  /* Green */
+.ruby-comment    { color: #6a737d; }  /* Gray */
+.ruby-regexp     { color: #032f62; }  /* Navy */
+.ruby-value      { color: #005cc5; }  /* Blue */
+.ruby-string     { color: #032f62; }  /* Navy */
+
+code {
+  background-color: #f0f4f8;  /* Match pre background */
+  padding: 0.2em 0.4em;
+  border-radius: 3px;
+  font-size: 85%;
 }
 
 em {
@@ -695,30 +724,6 @@ main .attribute-access-type {
 
 /* @end */
 
-/* @group Source Code */
-
-pre {
-  margin: 0.5em 0;
-  border: 1px dashed #999;
-  padding: 0.5em;
-  background: #262626;
-  color: white;
-  overflow: auto;
-}
-
-.ruby-constant   { color: #7fffd4; background: transparent; }
-.ruby-keyword    { color: #00ffff; background: transparent; }
-.ruby-ivar       { color: #eedd82; background: transparent; }
-.ruby-operator   { color: #00ffee; background: transparent; }
-.ruby-identifier { color: #ffdead; background: transparent; }
-.ruby-node       { color: #ffa07a; background: transparent; }
-.ruby-comment    { color: #dc0000; background: transparent; }
-.ruby-regexp     { color: #ffa07a; background: transparent; }
-.ruby-value      { color: #7fffd4; background: transparent; }
-
-/* @end */
-
-
 /* @group search results */
 #search-results {
   font-family: Lato, sans-serif;