GUI -- Added dark theming for the topology view.

Change-Id: I13c7bbbff65007fed162a4ea80dbf6042ff04c07
diff --git a/web/gui/src/main/webapp/app/onos.css b/web/gui/src/main/webapp/app/onos.css
index 1ab5954..0449853 100644
--- a/web/gui/src/main/webapp/app/onos.css
+++ b/web/gui/src/main/webapp/app/onos.css
@@ -37,6 +37,13 @@
     overflow: hidden;
 }
 
+body.light {
+    background-color: white;
+}
+body.dark {
+    background-color: #2b2b2b;
+}
+
 #view {
     padding: 6px;
     width: 100%;