UPDATE (04/09/2017): This issue appears to be fixed in Istanbul Patch 3 If your organization is using CMS and you are considering upgrading to Istanbul, be on the lookout for a potential issue. Theme CSS styles may not be applied to iFrame content in the CMS for some instances after upgrading to Istanbul. ServiceNow is already looking into the issue so hopefully we will have a resolution soon but I know of a few instances out there that might find this to be a deal breaker for an upgrade so I thought it best to advise the Community.