In the KV file, define the rule for the root widget.

Define a button with a label and a position.

In canvas.before, push the matrix onto the context's matrix stack.

Rotate the matrix 45 degrees.

Use the translate instruction to translate the matrix.

In canvas.after, pop the matrix from the context's matrix stack onto the model view:

<MyW>:
    Button:
        text: 'hello world'
        size_hint: None, None
        pos_hint: {'center_x': .5, 'center_y': .5}
        canvas.before:
            PushMatrix
            Rotate:
              ...