fix: restore dark mode body image backgrounds

Apply light gray backgrounds to body images in dark themes for both English and Chinese mdBook themes while explicitly excluding homepage frontpage images.

tests/test_dark_mode_images_css.py

@@ -0,0 +1,30 @@
+from __future__ import annotations
+
+import unittest
+from pathlib import Path
+
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+
+
+class DarkModeImagesCssTests(unittest.TestCase):
+    def test_both_theme_css_files_style_dark_mode_body_images_only(self) -> None:
+        css_paths = [
+            REPO_ROOT / "theme" / "dark-mode-images.css",
+            REPO_ROOT / "books" / "zh" / "theme" / "dark-mode-images.css",
+        ]
+
+        for css_path in css_paths:
+            css = css_path.read_text(encoding="utf-8")
+
+            self.assertIn(".navy .content main img", css, css_path.as_posix())
+            self.assertIn(".coal .content main img", css, css_path.as_posix())
+            self.assertIn(".ayu .content main img", css, css_path.as_posix())
+            self.assertIn("background-color: #e8e8e8;", css, css_path.as_posix())
+            self.assertIn(".openmlsys-frontpage img", css, css_path.as_posix())
+            self.assertIn("background-color: transparent !important;", css, css_path.as_posix())
+            self.assertIn("padding: 0 !important;", css, css_path.as_posix())
+
+
+if __name__ == "__main__":
+    unittest.main()