When using a theme for the Zabbix frontend that has a background color other than white, an external URL displayed in a screen may not display correctly.

Some simple websites assume a white background and do not explicit set the background color.
When using a Zabbix theme that has a background color other than white, the theme's background color is inherited by the external URL through the iframe used to load it. Depending on the design of the website this may make portions difficult or impossible to view.

Because the URL may be outside the control of the Zabbix user, this problem should be handled by Zabbix rather than modification of the page returned at the URL.

The URL we found the problem on is http://status.apps.rackspace.com/

The workaround I have found is to explicitly define a white background for the iframe in the theme's main.css. The line I have added to the dark themes is:
.screenitem iframe