Add theming info to README.

README.md

@@ -39,6 +39,41 @@ root folder:
 $ pip install -r requirements.txt
 ```
 
+Theming
+-------
+
+The Drag and Drop XBlock ships with an alternate theme called "Apros"
+that you can enable by adding the following entry to `XBLOCK_SETTINGS`
+in `lms.env.json`:
+
+```json
+        "drag-and-drop-v2": {
+            "theme": {
+                "package": "drag_and_drop_v2",
+                "locations": ["public/themes/apros.css"]
+            }
+        }
+```
+
+You can use the same approach to apply a custom theme:
+
+`"package"` can refer to any Python package in your virtualenv, which
+means you can develop and maintain your own theme in a separate
+package. There is no need to fork or modify this repository in any way
+to customize the look and feel of your Drag and Drop exercises.
+
+`"locations"` is a list of relative paths pointing to CSS files
+belonging to your theme. While the XBlock loads, files will be added
+to it in the order that they appear in this list. (This means that if
+there are rules with identical selectors spread out over different
+files, rules in files that appear later in the list will take
+precedence over those that appear earlier.)
+
+Finally, note that the default (unthemed) appearance of the Drag and
+Drop XBlock has been optimized for accessibility, so its use is
+encouraged -- especially for courses targeting large and/or
+potentially diverse audiences.
+
 Enabling in Studio
 ------------------