Microsoft released a new set of tools on Monday to help developers make slick-looking add-ins for Office apps, at a time when the company wants to boost the ecosystem of third-party applications and services that work with its productivity suite.

The new Office UI Fabric, which was released as an open-source project on GitHub, provides a set of tools that lets developers easily make their add-ins look more like the native Office user interface. Those add-ins allow companies like Salesforce, Smartsheet and Uber to bring their apps and services into Office apps, including Outlook and Excel, across a variety of devices including PCs, Macs and iPads.

Developers create add-ins using HTML and CSS. Prior to Monday's launch, they needed to create their own CSS to handle style and layout of the content for their add-in, which was time-consuming and added to inconsistency between different add-ins. It reached a point where Microsoft's own extensions were inconsistently designed.

Office UI Fabric alleviated that by creating a single unified framework for add-in user interfaces. By making it available to outside developers, Microsoft is also helping people who had also been looking to the company for add-in design guidance. In addition to the new tools, Microsoft also launched a new set of user experience guidelines for Office add-ins that incorporate many of the principles behind UI Fabric.

Microsoft recommends developers use its user interface toolkit (including Office UI Fabric) to design add-ins, but doesn't require its use. If a company has an existing design language for its products, developers are "welcome to use it, as long as the end result is a harmonious experience for Office users."

Add-ins are an important component of Microsoft's plans for Office, especially as the company faces increased competition in the office suite market from startups and large companies alike. Allowing users to access other popular tools from within Office apps enhances the utility of both Microsoft's software and that of the third parties contributing add-ins.

The news comes ahead of Microsoft's official launch of Office 2016 for Windows. A leaked slide claimed that the software (which is in public beta) will be made available to consumers on September 22, and Microsoft is currently internally testing a release candidate of its new suite.