Maybe color stars in... yellow
It just impossible to see if stars are set or not with current scheme... I use 5 stars most of the time...
I experience way more gray album covers than yellow. Maybe as someone said, a fill color then a border color. This is a frequent problem.
How about stars with an outer and inner (contrasting) colour. Then they are visible on top of any background.
They actually were yellow before, but that has problems too (if the album art is very yellow, they would be difficult to see with yellow color too). It's a problem I have tried to solve for ages, but I can never seem to do it right. I'll keep it in mind! ;)