We have setup Portal for ArcGIS 10.3.1 and deployed a single window Operations Dashboard. When I launch the Web version of Operations Dashboard, I do not get the option to change the theme. Also the Expand and collapse button that should come at the lower right corner of the panel does not show. When I check in Google Chrome Developer tools, it shows an 404 error - Failed to load resource: the server responded with a status of 404 (Not Found) ... http://xxx/arcgis/apps/dashboard/js/dashboard/img/expand16.png
OK thanks. For Portal 10.3.1, the ability to switch themes in web is not available (it will be in 10.4). It was released for ArcGIS Online organizations in July.
The second issue with the icon is a build issue. When portal gets built, the file names have case sensitivity that may not have been updated correctly. We noticed this on some other icons as well. I have logged an internal issue with our developers to fix this issue for the 10.4 release. In the meantime, you can browse to that folder on the server and change the name from Expand16.png to expand16.png.