Several bug fixes and improvements (#945)

* Update Dockerfile for code debugging.

* Format Python code using Black.

* Improve dark theme by defining html classes for the figures, animations and cover images.

* Fix several glossary translation.

* Update a code comment.

* Fix climbing_stairs_backtrack: the pruning should not require the sorted choices list.

* Update the code of array and list traversal.

* Fix a rendering issue of README.md

* Update code of list traversal.

* Fix array_definition.png

* Update README.md

* Fix max_capacity_moving_short_board.png

* Fix array.dart

* Fix array.dart

* Fix array.dart

* Fix array.dart

overrides/stylesheets/extra.css

@@ -25,7 +25,7 @@
   --md-default-fg-color: #adbac7;
   --md-default-bg-color: #22272e;
 
-  --md-code-bg-color: #1D2126;
+  --md-code-bg-color: #1d2126;
   --md-code-fg-color: #adbac7;
 
   --md-accent-fg-color: #aaa;
@@ -43,6 +43,23 @@
   color: var(--md-default-fg-color) !important;
 }
 
+/* Figure class */
+.animation-figure {
+  border-radius: 0.63rem;
+  display: block;
+  margin: 0 auto;
+}
+
+/* Cover image class */
+.cover-image {
+  width: 28rem;
+  height: auto;
+  border-radius: 0.3rem;
+  box-shadow: var(--md-shadow-z2);
+  display: block;
+  margin: 0 auto;
+}
+
 /* Center Markdown Tables (requires md_in_html extension) */
 .center-table {
   text-align: center;
@@ -82,12 +99,6 @@
   text-transform: none;
 }
 
-/* Image align center */
-.center {
-  display: block;
-  margin: 0 auto;
-}
-
 /* font-family setting for Win10 */
 body {
   --md-text-font-family: -apple-system, BlinkMacSystemFont,