markdown_docs: Fix styling for in-app docs for inline code.

This adds a selector `#message-formatting` which is basically the
modal id to apply night mode style to inline code in the modal.

static/styles/night_mode.scss

@@ -517,6 +517,7 @@ on a dark background, and don't change the dark labels dark either. */
     .message_edit_content code,
     .preview_content code,
     #settings_page code,
+    #message-formatting code,
     .typeahead.dropdown-menu {
         background-color: hsl(212, 25%, 15%);
         border-color: hsla(0, 0%, 0%, 0.5);