* Zoom in a bit
* Add Vertex [5] where you think the center of the rectangle is and set a vertex
* Opacity 25%
* Select black color
* Add primitive -> Rectangle -> Solid
* Draw black rectangle using the vertex you added previously as center
* Select white color
* Add primitive -> Rectangle -> Solid
* Draw white rectangle using the vertex you added previously as center
Screenshot and come back.
w.
LEGO ergo sum