Styling backgorund

The background is transparent now. For some user who use chrome-dev-tool dark theme, text is hardly recognizable. So change it in this PR.

Author: jaychsu

panel/app.css

@@ -1,3 +1,7 @@
+body {
+  background-color: rgb(255,255,255);
+}
+
 .col {
   float: left;
   width: 200px;