This is a pretty varied request of features. There isn't really a single thing that handles them all.
Godot has support for gathering a fair amount of data about the PC but I doubt it will be enough for what you want. You can look into Godot's built-in global classes first. For example OS, and VisualServer are good places to start. They have some functions to get stuff like the OS time, the GPU vendor and other basic info. I believe the OS class has a way to open a shell (aka, execute a program for you). You just need to dig around to find what you need.
Can't help you with the theme as that is different for every OS (and, if in linux, generally for each distro). You'd also have to find a way to convert from the system's native format into Godot's format.
Basically, if Godot can't do it natively then you need to plug into a language that has the ability to do what you want. This is where GDNative comes in as you can program this functionality in something like C++ or whathaveyou and let Godot call these functions. You'd need to know how to implement these features through another language, however, as well as make a dll, so, or dylib (depending on your OS) to pass around with your game.
