I've managed to change the color of rating stars on the main panel, main player panel, and the tag editor window. However, I haven't found a way to change the color of the stars in the "Details" section of the "Now Playing" panel on the right sidebar.
So far I've tried:
Panel.StarRating.Default
Panel.StarRating.Highlight
Panel.StarRating.Lowlight
PlayerFlat.StarRating.Default
Controls.StarRating.Default