Not the latest news, but I am more than happy to see it. The MS Office 2016 will feature dark themed user interface, which apparently was “the most requested feature for Office 2013”, see theverge.com.
If you want you may check it yourself. There is a public preview version available for free from Microsoft’s website. If you want different language version than your native language, you have to change the default language of your browser. In my case I wanted to use English version as I hate Polish translation. Interestingly, the English website did not contain 64-bit installation, but when I loaded Swedish website I noticed that the link stayed the same as the one I got from the Polish version in another browser. So, change the language of the browser to the desired one and get the installation file from http://go.microsoft.com/fwlink/p/?LinkID=534760
If for some reason the installation does not finish properly, check Control Panel > Programs and Features and if it is there uninstall and start the installation again.
To enable dark theme go to any MS Office program, e.g. Excel, then File > Options > General and there you will find a drop down menu named Office Theme, see the screen below.