added style for "demos" similar to "lectures"

assets/css/style.scss

@@ -88,6 +88,14 @@ dl dt.lecture {
     padding:5px;
 }
 
+dl dt.demonstration {
+    style:bold;
+    font-size: large;
+    background-color:   $lecture-background;
+    color: $white;
+    padding:5px;
+}
+
 dl dt.holiday {
     style:bold;
     font-size: large;