one-dark/gtk-3.0/sass/ui/lightdm.scss

// the panel widget at the top
#panel_window {
  background-color: $panel_bg;
  color: $panel_fg;
  font-weight: bold;
  box-shadow: inset 0 -1px darken($panel_bg, 7%);

  // the menubars/menus of the panel, i.e. indicators
  menubar,
  menubar > menuitem {
    background-color: transparent;
    color: $panel_fg;
    font-weight: bold;
  }

  menubar menuitem:disabled {
    color: transparentize($panel_fg, 0.5);

    label {
      color: inherit;
    }
  }
  menubar menu > menuitem {
    font-weight: normal;
  }
}

// the login window
#login_window,
#shutdown_dialog,
#restart_dialog {
  font-weight: normal;
  border-style: none;
  background-color: transparent;
  color: $fg_color;
}

// the top half of the login-window, in GtkDialog terms, the content
#content_frame {
  padding-bottom: 14px;
  background-color: $bg_color;
  border-top-left-radius: 2px;
  border-top-right-radius: 2px;
  border: solid transparentize(black, 0.9);
  border-width: 1px 1px 0 1px;
}

#content_frame button {
  @include button(normal);

  &:hover {
    @include button(hover);
  }
  &:active,
  &:checked {
    @include button(active);
  }
  &:disabled {
    @include button(insensitive);
  }
}

// the lower half of the login-window, in GtkDialog terms the buttonbox or action area
#buttonbox_frame {
  padding-top: 20px;
  padding-bottom: 0px;
  border-style: none;
  background-color: $header_bg;
  border-bottom-left-radius: 3px;
  border-bottom-right-radius: 3px;
  border: solid transparentize(black, 0.9);
  border-width: 0 1px 1px 1px;
  box-shadow: inset 0 1px $header_border;
}

#buttonbox_frame button {
  @include button(osd);

  &:hover {
    @include button(osd-hover);
  }
  &:active,
  &:checked {
    @include button(osd-active);
  }
  &:disabled {
    @include button(osd-insensitive);
  }
}

#login_window #user_combobox {
  color: $fg_color;
  font-size: 13px;

  menu {
    font-weight: normal;
  }
}

// the user's avatar box
#user_image {
  padding: 3px;
  border-radius: 2px;
}

// the border around the user's avatar box
#user_image_border {
}

// the shutdown button
#shutdown_button.button {
  @include button(suggested_destructive, $destructive_color);

  &:hover {
    @include button(suggested_destructive, lighten($destructive_color, 10%));
  }
  &:active,
  &:checked {
    @include button(suggested_destructive, darken($destructive_color, 10%));
  }
}

// the restart button
#restart_button.button {
  @include button(suggested_destructive, $suggested_color);

  &:hover {
    @include button(suggested_destructive, lighten($suggested_color, 10%));
  }
  &:active,
  &:checked {
    @include button(suggested_destructive, darken($suggested_color, 10%));
  }
}

// the warning, in case a wrong password is entered or something else goes wrong according to PAM
#greeter_infobar {
  border-bottom-width: 0;
  font-weight: bold;
}

//
// Lightdm
//

window.background.lightdm {
  menubar {
    > menuitem {
      padding: 0 5px 0 5px;
    }
  }

  button {
    &.flat.option-button {
      background-color: transparent;
      color: #dadada;

      &:hover {
        border-color: transparentize($borders_color, 0.8);
        background-color: transparentize(black, 0.5);
      }

      &:active {
        border-color: $borders_color;
        background-color: $selected_bg_color;
        color: $selected_fg_color;
      }
    }

    &,
    &.toggle-button {
      border-color: transparent;
      background-color: transparent;
      color: #dadada;

      &:hover {
        border-color: transparentize($borders_color, 0.8);
        background-color: transparentize(black, 0.5);
      }

      &:active,
      &.selected {
        border-color: transparentize($borders_color, 0.8);
        background-color: $selected_bg_color;
        color: $selected_fg_color;
      }
    }
  }

  entry,
  .lightdm-combo entry {
    background: transparentize(black, 0.5);
    color: white;
  }

  entry:active {
    // for spinner
    -gtk-icon-source: -gtk-icontheme('process-working-symbolic');
    animation: spinner 1s linear infinite;
  }

  .lightdm-combo {
    border-radius: 0;
    background-color: transparent;
  }
}