book2: use CSS variables for code colors to fix dark-mode contrast

Book pages used hard-coded colors for inline and block code in book2.scss,
which resulted in poor contrast in dark mode.

Replace those hard-coded values with CSS variables and override the
variables in dark mode, allowing the existing theme mechanism to handle
light and dark appearances consistently.

Author: Subhajit-das21

assets/sass/book2.scss

@@ -1,4 +1,9 @@
 .edition2 {
+    /* Book code colors (light mode defaults) */
+  --book-code-color: #333;
+  --book-code-bg: #eee;
+  --book-code-border: #f5f5f5;
+
   * {
     box-sizing: border-box;
   }
@@ -474,11 +479,11 @@
   }
 
   pre {
-    background: #eee;
+    background: var(--book-code-bg);
 
     code {
       display: inline;
-      background: #eee;
+      background: var(--book-code-bg);
       border: 0;
     }
   }
@@ -488,14 +493,14 @@
     overflow-x: auto;
     font-size: 0.9em;
     line-height: 120%;
-    color: #333;
+    color: var(--book-code-color);
   }
 
   code { /* inline code */
     padding: 0;
-    color: #333;
-    background: #eee;
-    border: 1px solid #f5f5f5;
+    color: var(--book-code-color);
+    background: var(--book-code-bg);
+    border: 1px solid var(--book-code-border);
   }
 
   table pre { /* code blocks within tables */

assets/sass/dark-mode.scss

@@ -117,14 +117,6 @@
                 background-color: #5e5951;
 
             }
-            /* Book pages: fix code contrast in dark mode */
-            .book code {
-                color: var(--fixed-width-font-color);
-            }
-
-            .book pre {
-                color: var(--fixed-width-font-color);
-            }
 
             form#search {
                 @include box-shadow(none);
@@ -149,6 +141,11 @@
             .cheat-sheet img {
                 filter: invert(60%);
             }
+
+            /* Book pages: code colors */
+            --book-code-color: #{$fixed-width-font-color};
+            --book-code-bg: #{$no-changes-bg-color};
+            --book-code-border: #{$pre-border};
         }
     }