As of now here's a quick schematic for the video shared on Reddit (post embedded below!). I will re-wrote this post in detail when I have time.
Reduce the value of C1 for faster tremolo or turn the potentiometer!
You can remove D1 once you've finished testing. The way in which circuit works is quite simple- The 555 produces a square wave in which the ON time is greater than the OFF time, this signal is inverted by an NPN transistor in the next section. And then finally, whenever the last LED(D2) glows, the LDR's resistance decreases and almost shorts the signal creating a superb tremolo effect!