This base is just a UI, there is no code in the main interface besides the Combo and Proxy open buttons.
The theming was possible by WPFDarkTheme made by AngryCarrot789
Simply download or clone the github repo and open the .sln file in Visual Studio 2019.
Then, go to Project > Manage NuGet Packages and update Fody.
Here's a picture of the UI:
If you do end up using this UI for any one of your projects, no need to give credit, it's all good.