I need to know whether its possible to customise the look and feel of IBM Mobile First App Center. Like adding our own branding image and changing the ui style of the App Center.
If you are referring to the Application Center Management Console - you cannot alter its UI because it is part of a .war file. You could potentially open this .war file, edit the files and re-deploy it. But this is an action not supported by IBM.
If you are referring to the Application Center Installer application that you install on the end-user device, yes, you can customize its look & feel, as you have the project source code. Feel free to alter its UI however you'd like. However if you alter its functionality, this will not be supported.