@daybugging You can store values in your config.php with c::set(). These values can then be retrieved using c::get(). An alternative would be a function in your plugins folders that returns a particular value and which you can call from anywhere. There is no such thing as a global controller.
Depending on your use case, you can use a default controller, but its use is pretty restricted, i.e. the default controller only works for pages with no page specific controller.