Add Background Color to Code Block Editor

- Introduced a new CSS rule to set the background color for the code block editor, enhancing visual distinction and user experience.
- This change improves the overall aesthetics of the editor interface, aligning it with the application's design standards.

assets/stylesheets/App.css

@@ -1680,3 +1680,7 @@ span.ant-skeleton-input.ant-skeleton-input-sm.text-skeleton {
   margin: 0;
   font-size: 95%;
 }
+
+.code-block-editor .simplebar-content-wrapper {
+  background-color: var(--layout-modal-bg);
+}