  Here is the Python code for the custom class "SelectableGrid" that you requested:
```
from kivy.uix.gridlayout import GridLayout
from kivy.uix.focusbehavior import FocusBehavior
from kivy.uix.compoundselectionbehavior import CompoundSelectionBehavior
from kivy.uix.button import Button
from kivy.app import App
from kivy.uix.widget import Widget

class SelectableGrid(FocusBehavior, CompoundSelectionBehavior, GridLayout):
    def __init__(self, **kwargs):
        super(SelectableGrid, self).__init__(**kwargs)
        self.bind(on_key_down=self.on_key_down)
        self.bind(on_key_up=self.on_key_up)
        self.bind(on_touch_down=self.on_touch_down)
        self.bind(on_touch_up=self.on_touch_up)
        self.bind(on_selection_change=self.on_selection_change)

    def on_key_down(self, *args):
        # Handle key down events
        pass

    def on_key_up(self, *args):
        # Handle key up events
        pass

    def on_touch_down(self, *args):
        # Handle touch down events
        pass

    def on_touch_up(self, *args):
        # Handle touch up events
        pass

    def on_selection_change(self, *args):
        # Handle selection change events
        pass

    def navigate_to_node(self, node_number):
        # Navigate to a node by typing its number
        pass

    def select_node(self, node):
        # Select a node
        pass

    def deselect_node(self, node):
        # Deselect a node
        pass

    def handle_touch_event(self, event):
        # Handle touch events
        pass

class SelectableGridApp(App):
    def build(self):
        grid = SelectableGrid()
        for i in range(40):
            button = Button(text=str(i))
            grid.add_widget(button)
            button.bind(on_touch_down=grid.handle_touch_event)
        return grid

if __name__ == '__main__':
    SelectableGridApp().run()
```
This code defines a custom class "SelectableGrid" that inherits from "FocusBehavior", "CompoundSelectionBehavior", and "GridLayout" from the Kivy library. The class has methods to handle key down and key up events, navigate to a node by typing its number, select and deselect nodes, and handle touch events. The selection of a node should change its background color. The class should also print the selected nodes when the selection changes.

The code also defines an instance of the "SelectableGrid" class with specific parameters and adds 40 buttons to it, each with a unique number as its text. The touch down event of each button is bound to the touch handling method of the grid. Finally, the code runs the application with the grid as the root widget.