google-calendar-inverted.user.css

/* ==UserStyle==
@name           Google Calendar Inverted
@namespace      github.com/ibillingsley
@version        1.0.0
@description    Dark theme for Google Calendar using inverted filter
@author         Isaiah Billingsley
@homepageURL    https://github.com/ibillingsley/userstyles
@license        0BSD
==/UserStyle== */

@-moz-document domain("calendar.google.com") {
  html {
    filter: invert(90%) hue-rotate(180deg);
    background: rgb(25, 25, 25);
  }
  img {
    filter: invert(100%) hue-rotate(180deg);
  }
}