Microsoft has released Fluent XAML Theme Editor to the Microsoft Store. Fluent XAML Theme Editor is a design development tool developers can use to create custom themes for UWP apps. This tool also helps with the development flow by generating and exporting your chosen theme to a XAML markup resource dictionary.
In addition, the tool and its source code are public and can be seen at GitHub here.
The tool requires Visual Studio 2017 Update 4 or higher and the Windows Software Development Kit (SDK) version 17763 or higher for Windows 10.
Find it in the Store here.