You just need to add two additional lines to the colors file.
Look for the line atom-bluebg-noalpha and add atom-blugebg-noalpha-classical beneath it. Set the hex value to the same for both, i.e. copy the hex value from atom-bluebg-noalpha
Add a new line at the end of the file as follows:
album-dimlayer
Again, copy the hex value from the layer above (I.e. the dimlayer hex value).
For example, for one of my light themes, I now have the following additional lines:
atom-bluebg-noalpha-classical #888888
and,
album-dimlayer #70%181818