added code mirror styles

common/lib/xmodule/xmodule/css/codemirror/codemirror.scss

+.CodeMirror {
+  background: #fff;
+  font-size: 13px;
+  color: #3c3c3c;
+}
\ No newline at end of file

common/lib/xmodule/xmodule/editing_module.py

@@ -30,6 +30,8 @@ class XMLEditingDescriptor(EditingDescriptor):
     any validation of its definition
     """
 
+    css = {'scss': [resource_string(__name__, 'css/codemirror/codemirror.scss')]}
+
     js = {'coffee': [resource_string(__name__, 'js/src/raw/edit/xml.coffee')]}
     js_module_name = "XMLEditingDescriptor"
 
@@ -40,5 +42,7 @@ class JSONEditingDescriptor(EditingDescriptor):
     any validation of its definition
     """
 
+    css = {'scss': [resource_string(__name__, 'css/codemirror/codemirror.scss')]}
+
     js = {'coffee': [resource_string(__name__, 'js/src/raw/edit/json.coffee')]}
     js_module_name = "JSONEditingDescriptor"