Hi,
The My Inbox Fiori app reads the approve/reject button properties from the decisions configured in customizing in the back-end system as given here App Implementation: My Inbox - SAP Fiori for Request Approvals 1.0 - SAP Library under Configuring a Task and Decisions.
Are you first able to see the Approve and Reject buttons in green and red color in the standard My Inbox app?
Regards,
Saurabh