Use this link for faster performance: https://turbowarp.org/723664608 How this works is it forever activates the custom block named draw. In this block it repeats (var) hight activate the custom block randomize, set x to 0, set (var) i to 0, and it repeats (var) width the system which draws the box. That is a (brief) explanation on how this works.
100% by @-Voxel- (me). Btw, it has a very limited RGB set.