You're referring to this?:
https://developer.mozilla.org/de/docs/Web/JavaScript/Reference/Errors/For-each-in_loops_are_deprecatedI searched the add-on code for deprecated "for each" loops but found nothing. So I understand that this is not the problem.
Are the colors rendered wrong in the existing color-edits or only during/after new color edits?
Wrong colors in already existing edits indicate "problems" or "behaviour changes" in the interpretation of CSS rules by mozilla. The add-on just creates static user-defined css. When Mozilla "mis-interpretes" such edits it might be either an error in their CSS-engine (it is a nightly build we're speaking of after all) or there was some CSS error/inconsitency for ages in the engine which is now "corrected".