I am assuming you are admin for the group. If not please get the permission of admin.
Then go to "manage" menu item(top right)>> settings . Click on the banner design button (will appear on the banner top right corner). I believe this will solve your purpose .
The Manage > Settings and clicking Banner Design is the method that I used to change my banner background. There is no option to change the text color there except in the Custom tab.
The text color appears to change to grey right after I click the Banner Design button.