This style is based on the Microsoft Universal Design Guidelines.
The colors and metrics used all around the QML and C++ files originate from the Windows 10 SDK. The files are called generic.xaml and themeresources.xml, and they are located in the following folder:
\(Program Files)\Windows Kits\10\DesignTime\CommonConfiguration\Neutral\UAP\<SDK version>\Generic
See also XAML theme resources.