book: fix code styling in dark mode

Book pages used hard-coded colors for inline and block code (e.g.
`#eee`, `#333`) in `book2.scss`, which prevented dark-mode styles from
taking effect. As a result, code blocks in the Git Book did not adapt
correctly when switching themes.

This change replaces those hard-coded values with CSS variables
(`--book-code-bg`, `--book-code-color`, `--book-code-border`), allowing
dark mode to override Book code styling in the same way as other
sections (e.g. man-pages).

Not all `code.<language>` color rules are adjusted here, as they
relate to syntax highlighting rather than theme contrast and require a
separate, more holistic approach.

An alternative approach—overriding Book code styles directly in
`dark-mode.scss` using more specific selectors—was considered but
rejected, as it would further increase selector complexity instead of
fixing the root cause.

Fixes #2120

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: Subhajit-das21

assets/sass/book2.scss

@@ -1,4 +1,8 @@
 .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 +478,11 @@
   }
 
   pre {
-    background: #eee;
+    background: var(--book-code-bg);
 
     code {
       display: inline;
-      background: #eee;
+      background: var(--book-code-bg);
       border: 0;
     }
   }
@@ -488,14 +492,14 @@
     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

@@ -83,6 +83,13 @@
                 z-index: -1;
             }
 
+            /* Book pages: code colors */
+            .edition2 {
+                --book-code-color: #{$fixed-width-font-color};
+                --book-code-bg: #{$no-changes-bg-color};
+                --book-code-border: #{$pre-border};
+            }
+
             img {
                 filter: brightness(.6) contrast(1.2);
             }