Deal with the Darcula color scheme

[eugenschindler]

Nowadays, the Darcula color scheme is the default in MPS and plaintextgen has trouble with the Darcula color scheme in some cases. The only example I have right now is the color of the alignment icons: ◧, ◨, and ◫ (which are too dark in Darcula). It would be good to inventory which elements make contrast difficulty in the Darcula scheme and add Darcula-specific colorings for those elements.