markdown preview: Fix `code` style for preview in night mode.

When in night mode, `code` style was still the same as in the light mode;
The fix is for the same.

Fixes #11269.

static/styles/night_mode.scss

@@ -484,6 +484,7 @@ on a dark background, and don't change the dark labels dark either. */
     #feedback_container,
     .message_content code,
     .message_edit_content code,
+    #preview_content code,
     #settings_page code,
     .typeahead.dropdown-menu {
         background-color: hsl(212, 25%, 15%);