From Ren'Py Visual Novel Engine

Centered Window

The following code should be put in one of your RPY files, preferably options.rpy. If you are not sure where to put it, place it at the very end of your file.

init python hide:
    import os
    os.environ['SDL_VIDEO_CENTERED'] = '1'

This forces the main program window to be centered. Without this code, the placement of the window is left up to the OS.