Best Answer Danny , 21 November 2013 - 02:16 PM
You do not require any custom CSS so you can remove James suggested code, your issue is related to your layout choice.
Go to Wordpress Admin Dashboard > PlatformPro > Design Control and select Full-Width Design with Canvas. This should give you a background image (which you shoudl have uploaded) and a content area with the color you have selected.
Also, please be in mind that Platform and Platform Pro have been considered legacy themes for sometime now and with that, we do not provide support any more. If you wish to continue using Platform that is entirely up to you, but support is no longer provided. I would recommend you upgrade to DMS Free.