theme_custom_0_x
The option documentation is properly formatted yet
Example: 1605
The X coordinate for the custom overlay image. Important note: the coordinates are relative to a 1920x1080 coordinate system with (0,0) in the top-left corner, even though your window may have another size. This means that the center of the window is always coordinate (960, 540).
Source: https://github.com/FrodeSolheim/fs-uae/blob/master/docs/options/theme_custom_0_x