Here's how you do it:
1) Go to : AwareIM/Tomcat/Webapps/Start/ext20/resources/CSS/
2) Open the CSS file related to the theme you want to change: eg x-theme-gray
3) search for "x-tab-panel-body {" then change the background colour there to whatever you want it to be (a tip is to use Firebug or other browser colour picker add-in to pick the colours to get the match exactly right)
But just as Support said above... beware when you do this you are changing the colour for that form across all business spaces PLUS when you upgrade aware you will need to do this change again.