Pretty easy fix...not sure why it still isn't implemented on this latest release of hustle so want to double the developer recognizes the issue...because obviously the message didn't get through. This fix should take no more than 5 minutes.
To repeat the bug.
Go to hustle in dashboard
edit a hustle opt in
change the background color (pick any of them) to blank by clicking the red x button
press next until back to the hustle list of optins
edit the optin changes will not be saved.
Developer is blocking the condition that background color elements can be left blank to allow for transparency with PNG files.
Again...this is a bug and needs to be addressed (and can be fixed very very quickly)