userChrome.css

/* Basic Transparency */
:root {
  @media not (prefers-reduced-transparency) {
    appearance: -moz-sidebar !important;
  }

  --dmn-theme-main-background-light: transparent;
  --dmn-theme-main-background-dark: rgba(0, 0, 0, 0.1);
  --dmn-theme-main-background: var(--dmn-theme-main-background-light);
}

@media (prefers-color-scheme: dark) {
  :root {
    --dmn-theme-main-background: var(--dmn-theme-main-background-dark);
  }
}

:root[lwtheme-brighttext="true"] {
  --dmn-theme-main-background: var(--dmn-theme-main-background-dark);
}

/* Transparent Styling */
:root:not([windowtype="Toolkit:PictureInPicture"], [inDOMFullscreen]) {
  #browser,
  #browser :is(#sidebar-main, #tabbrowser-tabbox),
  #navigator-toolbox,
  #navigator-toolbox > * {
    background: var(--dmn-theme-main-background) !important;
  }
}