Return it under guarantee to the shop and get a replacement or refund, there is nothing you can do to fix it yourself. It is a fault with the light controller in the case and the only way to sort it out is to replace it.
Gimmicks like these often go wrong and there is little anyone can do to fix them. I had a case with a lit up temp/time display which stopped working properly just before the gurantee ran out, when I contacted the company to get a replacement they agreed at first but took their time to send a replacement. They took so long over it that the gurantee expired and they refused any contact after that.
Better to get it sorted out ASAP, unless you don't mind the effect you have now.