While we have new icons for the desktop, it looks like the modern UI icons are being overhauled as well. Thanks to a tip, we can now see the old icons and the new icons side-by-side; the old icons are the darker gray whereas the new icons are in white.
The icons are thinner and generally void of any color fill as well; it looks like nearly every modern UI icon is being updated with Windows 10. The file where these icons were found is called MDL2 Asset which could stand for "modern design language 2".
We have already seen some of these icons show up in 9901, which adds to the credibility that all of these icons will be present in Windows 10.
For those who have been waiting for the UI elements of Windows 10 to be updated, it looks like this is part of the development process is well underway. While this build was not intended to be seen by those outside of Redmond, it does paint a good picture of what to expect in late January when the next build will be released.
Thanks for the tip Martin!