The Flat UI Light VCL Style is part of the new premium styles introduced in 10.3.1. Once installed you will find this file in your styles folder:
To install Flat UI Light VCL Style, just make sure you are updated to 10.3.1, launch RAD Studio, then go Tools ➜ GetIt Package Manager, and you will find VCL Style - Flat UI Light 1.0.
You can apply the styles to your VCL applications by visiting Project ➜ Options ➜ Application ➜ Appearance, and adding the style to your project, and optionally setting it as the default. At runtime, you can change styles with the code:
Here is a preview.