Light Up: Night mode instructions wording

[HannahPoy]

Instructions should be adjusted so that they do not misdirect in dark mode. (Here the black squares are the squares referred to as non-black in the instructions)

[chrisboyle]

There are several other examples of this kind of problem in the documentation, where e.g. upstream's lack of night mode means the docs talk about exact colours in this way, and the controls are obviously different, which I've bodged around by adding the italicised section showing at the bottom of the above screenshot.

The hard part is that I would very much like to avoid having the docs substantially rewritten for Android, because it makes it difficult to merge future changes from upstream, and in particular it would impede my ability to reuse translations from other platforms.