Select the fields and make the background blue. Set the text to white. Then add conditional format to the fields (you can add it to all the fields at once). Formula is 1. Set the text to black and the background to light grey (or what your standard default colors are when the fields are NOT currently selected).
Since conditional format on a field can only happen when you are not in the field, you need to apply reverse thinking, i.e. the format does not apply when you are in the field (blue with white text) but does apply when you aren't (grey with black text) .
Instead of transparent, set the conditional background color to your regular background instead.