Hello Karl,
I unpacked the zip you attached. It is probably incomplete compared to the GitHub link.
Basically the WPF starts in MainWindow.xaml where a user control is defined and from TessUiController.cs the control is filled with a login control. In the Style folder, they also define their own layout of controls like the Textbox, Slider, etc in terms of color, border etc.
I think WPF leaves much to be desired, especially in terms of how the design view interacts with the code. For example, I have a WPF ribbon (you can see how it looks like in the -yet to be finished- website photorganize.eu) and it is not possible to see the 2nd and 3rd tab in design view, as far as I know. Also, placing a control somewhere in the designer often doesn't turn out to show where you placed/expect it. In the meantime I have a bag of tricks to try solving that kind of issues but WPF can't be called programmers friendly. On the other hand, having the screens as Xaml code means, at least in theory, that you should be able to influence anything.
I am not sure what you read in ILSpy, but it should be either an .exe or a dll (not the zip). Unless it it obfuscated, which is not to be expected for a program with the sources on Github, you should see the code pretty much as it is in the original. But maybe they obfuscated one of the DLL's to prevent you from finding out how the access to a Tesla is done exactly.
If you have more (specific) questions, feel free to ask and I will reinstall from Github (if I don't need to login to get the full solution).
If you mail me your Tesla login credentials I could check if assigning 0 to the public int ChargeLimit (get/set) means you are unable to recharge your Tesla from then
Dick