gnome-shell/data/theme/gnome-shell-sass/widgets/_message-list.scss

/* Message List */
// a.k.a. notifications in the menu

// main list
.message-list {
  width: 31.5em;
  padding: 0 $base_padding * 2;
}

.message-list-sections {
  margin:$base_margin;
  spacing: $base_spacing;
}

.message-list-section,
.message-list-section-list {
  spacing: $base_spacing;
}

.message-list-section-list {
  &:ltr {padding:0;}
  &:rtl {padding:0;}
}

// clear button
.message-list-clear-button {
  margin:$base_margin $base_margin*2;
  .button {
    @extend %button;
  }
}

// message bubbles
.message {
  @extend %notification_bubble;

  // title
  .message-title {
    color: $fg_color;
    font-weight: bold;
    margin-bottom:4px;
  }

  // content
  .message-content {
    color: darken($fg_color, 10%);
    padding: $base_padding 0;
    margin:$base_margin * 2;
    &:ltr {
      margin-left: $base_margin;
      padding-right:$base_padding;
    }
    &:rtl {
      margin-right: $base_margin;
      padding-left:$base_padding;
    }
  }

  // close button
  .message-close-button {
    color: lighten($fg_color, 15%);
    &:hover { color: darken($fg_color, 10%);}
  }

  // icon container
  .message-icon-bin {
    padding: $base_padding;
    margin:$base_padding 0;

    &:rtl {
      // padding: $base_padding;
    }

    // icon size and color
    > StIcon {
      color: $fg_color;
      icon-size: $base_icon_size*2; // 32px
      -st-icon-style: symbolic;

      padding:0;
      margin:$base_padding;
    }

    // fallback
    > .fallback-window-icon {
      width: $base_icon_size;
      height: $base_icon_size;
    }
  }

  // secondary container in title box
  .message-secondary-bin {
    padding: 0;

    // notification time stamp
    > .event-time {
      color: transparentize($fg_color, 0.5);
      font-size: 80%;
      text-align: right;
      margin: 0 $base_margin * 2;
      /* HACK: the label should be baseline-aligned with a 1em label, fake this with some bottom padding */
      padding-bottom: $base_padding;
    }
  }

}

/* Media Controls */
.message-media-control {
  padding: $base_padding * 2 $base_padding * 4;
  color: darken($fg_color, 15%);

  // uses $hover_bg_color since the media controls are in a notification_bubble
  &:hover {
    background-color: lighten($hover_bg_color, 5%);
    color: $fg_color;
  }

  &:active { 
    background-color: darken($hover_bg_color, 2%);
    color: $fg_color;
  }

  &:insensitive { color: darken($fg_color,40%); }
  
  // fix border-radius for last button on hover
  &:last-child:ltr { &:hover {border-radius:  0 $base_border_radius+2 $base_border_radius+2 0;} }
  &:last-child:rtl { &:hover {border-radius: $base_border_radius+2 0 0 $base_border_radius+2;} }
}

// album-art
.media-message-cover-icon {
  icon-size: $base_icon_size*2 !important; // 48px
  border-radius: $base_border_radius;

  // when there is no artwork
  &.fallback {
    color: darken($fg_color, 17%);
    background-color: $bg_color;
    border: 1px solid transparent;
    border-radius: $base_border_radius;
    icon-size: $base_icon_size * 2 !important;
    padding: $base_padding * 2;
  }
}