Not sure what querySelectorAll is all about; can't we just call document.getElementsByTagName()?
So if an object says p.snork has bgcolor=white
then we get the array
a = document.getElementsByTagName("p");
Loop over array and if obj.class == "snork" then obj.style.bgcolor = white.
Or if the descriptor is on #instructions rather than a class of nodes, we use getElementById to find the node and then set its values.
So I think we already have the middle third, and the last third seems reasonably easy to write.

Karl Dahlke