Maximize current window completely to hide background in Macbook
I found this app BetterTouchTool. It lets me define a keyboard shortcut to maximize a window fully, without leaving black spaces on either sides.


Download app and open preferences by clicking its icon (boxed index finger) in the menu bar at the top of your screen.
Select 'Keyboard' tab
Select 'Global' in Applications on right
Finally choose Keyboard shortcut and Predefined Action.


I chose the following keyboard shortcuts, which I found safest and non overlapping:


Control ↑ Maximize Window
Control ← Maximize Window Left
Control → Maximize Window Right
Control ↓ Maximize window to bottom half