fix(styles): make operators visible in dark mode code listings

Add syntax highlighting color overrides for dark mode to fix invisible
operators (+=, +, *, =) and other code tokens. The light theme uses
nearly black colors (#1A1A1A) that blend into dark backgrounds.

Colors follow One Dark Pro theme palette:
- Operators: cyan (#56b6c2)
- Variables: light gray (#e6e6e6)
- Keywords: purple (#c678dd)
- Functions: blue (#61afef)
- Strings: green (#98c379)
- Numbers: orange (#d19a66)

Closes #1106

book/quarto/assets/styles/dark-mode.scss

@@ -328,6 +328,81 @@ pre code {
   padding: 0;
 }
 
+// Syntax highlighting colors for dark mode
+// These override the light theme colors that become invisible on dark backgrounds
+pre code,
+.sourceCode {
+  // Operators (+=, +, *, etc.) - bright cyan for visibility
+  .op {
+    color: #56b6c2 !important;
+  }
+
+  // Variables - light gray instead of black
+  .va {
+    color: #e6e6e6 !important;
+  }
+
+  // Keywords (def, for, if, etc.) - keep purple but brighten
+  .kw {
+    color: #c678dd !important;
+  }
+
+  // Control flow (return, break, etc.)
+  .cf {
+    color: #e06c75 !important;
+  }
+
+  // Functions - bright blue/purple
+  .fu {
+    color: #61afef !important;
+  }
+
+  // Built-ins - orange
+  .bu {
+    color: #e5c07b !important;
+  }
+
+  // Strings - green
+  .st {
+    color: #98c379 !important;
+  }
+
+  // Comments - muted gray
+  .co {
+    color: #7f848e !important;
+  }
+
+  // Numbers (decimal values, floats)
+  .dv, .fl {
+    color: #d19a66 !important;
+  }
+
+  // Imports
+  .im {
+    color: #61afef !important;
+  }
+
+  // Attributes
+  .at {
+    color: #e06c75 !important;
+  }
+
+  // Data types
+  .dt {
+    color: #e5c07b !important;
+  }
+
+  // Special characters
+  .sc {
+    color: #56b6c2 !important;
+  }
+
+  // Preprocessor
+  .pp {
+    color: #e5c07b !important;
+  }
+}
+
 // Figures and captions
 .figure-caption,
 .caption,
@@ -653,6 +728,21 @@ blockquote {
 
 .listing .sourceCode {
   background-color: #1e1e1e !important;
+
+  // Syntax highlighting within listings (higher specificity)
+  .op { color: #56b6c2 !important; }
+  .va { color: #e6e6e6 !important; }
+  .kw { color: #c678dd !important; }
+  .cf { color: #e06c75 !important; }
+  .fu { color: #61afef !important; }
+  .bu { color: #e5c07b !important; }
+  .st { color: #98c379 !important; }
+  .co { color: #7f848e !important; }
+  .dv, .fl { color: #d19a66 !important; }
+  .im { color: #61afef !important; }
+  .at { color: #e06c75 !important; }
+  .dt { color: #e5c07b !important; }
+  .sc { color: #56b6c2 !important; }
 }
 
 // Video containers