It's just lines, I use the dark grey and dark blue for the effect. The dark blue lines stop just below the middle of the image whilst the grey lines go to the bottom, this just adds a bit of depth to it all. I render them from an array of 128 points, I start them outside the top of the screen, and just move them right and down. When they hit the bottom, I recycle the line and put it at a random position at the top again. There is even a little blue pixel 'splash' for a frame when it hits the ground. Although it's not that much visible.
Here is the source code. If you want to use it: https://pastebin.com/C4fjNZkA