mobile-config-firefox/chrome/proton_dark_light_notifications.css

/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/proton_dark_light_notifications.css made available under Mozilla Public License v. 2.0
See the above repository for updates as well as full license text. */

/* Makes web notifications use dark/light theme based on your selected theme, and makes them fit a bit better to rounded proton style. */

@-moz-document url("chrome://global/content/alerts/alert.xhtml") {
  
  /* Color scheme definitions
   * Intentionally let Firefox override these on Windows10
   */
  :root{
    --menu-color: #15141a;
    --menu-background-color: #f9f9fb;
    --menu-border-color: #cfcfd8;
    --menuitem-hover-background-color: #e0e0e6;
  }
  @media (-moz-toolbar-prefers-color-scheme:dark) {
    :root {
      --menu-border-color: #5b5b66;
      --menu-color: #fbfbfe;
      --menu-background-color: #2b2a33;
      --menuitem-hover-background-color: #52525e;
    }
    #alertSourceLabel { color: rgb(5,209,241) !important; }
  }
  
  /* The line below removes background "layer" from the notification on linux, Windows and macOS don't have that layer in the first place */
  :root{ background: transparent !important; }
  
  /* Make Firefox actually use the colors set above */
  #alertBox {
    color: var(--menu-color) !important;
    background-color: var(--menu-background-color) !important;
    border-color: var(--menu-border-color) !important;
    border-radius: 6px !important;
    -moz-window-shadow: cliprounded !important;
  }
  #alertSettings {
    fill: currentColor !important;
    color: inherit !important;
    border-radius: 0 !important;
    margin-inline: 0 !important;
    margin-bottom: -4px !important;
  }
  .close-icon,
  #alertSettings {
    background: transparent !important
  }
  .close-icon:hover > .toolbarbutton-icon,
  #alertSettings:is(:hover,[open]) > .button-box > .box-inherit {
    background-color: var(--menuitem-hover-background-color);
  }
  
  /* Reshape and move the buttons a bit */
  .close-icon > .toolbarbutton-icon,
  #alertSettings > .button-box > .box-inherit {
    border-radius: 4px !important;
    padding: 2px !important;
    margin: 2px 2px -2px 0 !important;
  }
  #alertSettings > .button-box > .box-inherit {
    margin: -4px 4px 3px 0 !important;
  }
  #alertSettings > .button-box > .box-inherit > .button-icon {
    padding: 1px;
  }
}