Fix YANG Model details panel to have scrolling Source Div.

Change-Id: I23dd77ee13e95f73dcaae5d72f553fdfd837d732
diff --git a/apps/yang-gui/src/main/resources/app/view/yangModel/yangModel-theme.css b/apps/yang-gui/src/main/resources/app/view/yangModel/yangModel-theme.css
index afeb601..4d58ebd 100644
--- a/apps/yang-gui/src/main/resources/app/view/yangModel/yangModel-theme.css
+++ b/apps/yang-gui/src/main/resources/app/view/yangModel/yangModel-theme.css
@@ -14,3 +14,9 @@
  * limitations under the License.
  */
 
+.light #yang-model-details-panel .src-frame {
+    background-color: #f4f4f4;
+}
+.dark #yang-model-details-panel .src-frame {
+    background-color: #1b1b1b;
+}