Press F1 and a window will appear on the left side of the game. Check the box that says full screen and close the game. When you open it again you will see the game in full screen, it may turn off and turn on the monitor and change resolution, but that is only in the game. Then to get the game on its own and not as if it were in windowed mode, hit ALT+TAB, which makes you see what is open on the computer and then go back and hit it and enter the game. You will see how the borders appear in black.
Sorry if it is not well understood, but I am translating it from Spanish to English.