How to change the color of the box surrounding a Tkinter Button?
That white area is called the traversal highlight region. It changes color to let the user know when the button has the keyboard focus.
You can change its non-focused color with the highlightbackground option, and its focused color with highlightcolor. You can set it to the color of the background to get it to blend in.
empty.configure(highlightbackground="#009588")