That bugged me too. I see in the new version of LMMS 1.3 changes were made to the piano roll. Black and white keys are now drawn directly from the css code. I guess .png files for the white and black keys are no longer used.
Here is edited version of the Cameleon theme style.css file.
https://drive.google.com/file/d/1085FNU ... sp=sharing
Just overwrite the existing style.css file in Cameleon theme folder.