If the bark of trees is light in color, what do you expect to happen to the percentages of light and dark moths

You would see more lighter moths, due to natural selection. At the moment, the lighter moths have the more desirable gene so they are the ones who are going to spread it more.

As tree bark lightens you would expect to see more light coloured moths reaching maturity to reproduce. Their fitness would be increased due to an increased ability to camouflage against the tree in comparison to the darker coloured moths. As a result darker coloured moths would experience high rates of predation and reduced chances of reproduction. Eventually the moth population would shift to almost entirely light colouration due to increased levels of survival and fitness.