added highlight to element on hover by decreasing the opacity of the bac...

...kground elements using a global higlight function

@jebeck can you look at it, i think its ready but i'd like to hear what you think of the code and the resulting interaction.
@brandonarbiter @HowardLook @jebeck I  think we should think seriously about prioritising adding bg reading to this interaction. Its an internal part of the bolus wizard.