darker-gmail.user.css

/* ==UserStyle==
@name           Gmail - Darker Gmail
@description    Makes Gmail's inbox darker.
@version        2022.07.31.10.59
@author         MetalTxus
@namespace      https://github.com/jesuscc1993/
@license        CC BY-SA 4.0
==/UserStyle== */

@-moz-document domain("mail.google.com") {
  .bkK > .nH,
  .TD {
    background-color: #0008;
  }
}