userContent clobbers ProtonMail composer background

[proof-by-accident]

Raising this issue in case someone else has the same problem. The lines:

@-moz-document url(about:blank), url(about:newtab), url(about:home) {
    html:not(#ublock0-epicker), html:not(#ublock0-epicker) body, #newtab-customize-overlay {
      background: var(--mff-bg) !important;
    }
  }

cause the ProtonMail client composer to take on whichever color you've set as --mf-bg (in my base #000000). To fix I just commented out the line and the overall theme seems unbroken, but there may be a smarter fix (I'm crap at CSS, sorry).