Office UI Fabric is a fairly new web framework from Microsoft which lets you build applications that fit well within the 'Office 365' style.
It is especially useful for building Office 365 add-ins as it gives a more seamless experience to the user. There are a number of pre-built components you can take advantage of as well.
I have just released a gem that packages up the Office UI Fabric framework so you can easily include it into the asset pipeline within your Rails app.
More info can be found here: https://github.com/craigplummer/office-ui-fabric-rails
I'm currently building an office add-in that connects Outlook to Todoist and hope to blog about it soon as well as my use of Office UI Fabric.