IBM Lotus Domino 8.5.1 comes pre-installed with OneUI v2.01 for evaluation purposes.
OneUI v2.x is in use in other Lotus Products, and is pencilled in for Domino/XPages for version 8.5.2 (with the usual caveats of "this is in plan, however is not guaranteed and future plans are subject to change").
Please read the release note here
Together with releasing these files for evaluation, full documentation is also available which as well as detailing how to use the evaluation files, they may also help developers work with the existing OneUI files used by XPages.
You can access this documentation online here
The files can be found on the Server and Client within dominodata/domino/html/oneuiv2. In order to use these files, you can refer to them from your theme as follows:
Examples using these files will shortly be available on OpenNTF