Yesterday we announced the general availability of Office UI Fabric on GitHub. It's a neat new UI framework for making custom applications look and feel more like native O365 applications.
Office UI Fabric is a responsive, mobile-first, front-end framework for developers, designed to make it simple to quickly create web experiences using the Office Design Language. It’s simple and familiar to get up and running with Office UI Fabric—whether you’re creating a new app from scratch or adding new features to an existing one.
Head to the Office Blog for more details on the announcement and the GitHub code: https://blogs.office.com/2015/08/31/introducing-office-ui-fabric-your-key-to-designing-add-ins-for-office/