Solve the problem to make it black.
Level 7 depends on which device you are playing
on touchscreens you will find it out pretty soon (still depending on screen size not easy to solve)
with mouse it's very easy just try things out
with touchpad - could get you in trouble (game was made for touch), but 80% chance it will work
Hint resize/zoom
Spoiler for level 4:
click once
click twice
....