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-placeholder { spacing: 12px; }
}

.message-list-sections {
  spacing: $base_spacing;
  margin: 0 $base_margin * 4; // to account for scrollbar
}

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

// do-not-disturb + clear button
.message-list-controls {
  margin: ($base_margin * 2) ($base_margin * 4) 0;
  // NOTE: remove the padding if notification_bubble could remove margin for drop shadow
  padding: $base_margin;
  spacing: $base_spacing * 2;
}

// message bubbles
.message {
  @include notification_bubble;

  // icon container
  .message-icon-bin {
    padding: ($base_padding * 3) 0 ($base_padding * 3) ($base_padding * 2);

    &:rtl {
      padding: ($base_padding * 3) ($base_padding * 2) ($base_padding * 3) 0;
    }

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

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

  // content
  .message-content {
    padding: $base_padding + $base_margin * 2;
    spacing: 4px;
  }

  // title
  .message-title {
    font-weight: bold;
  }

  // secondary container in title box
  .message-secondary-bin {
    padding: 0 $base_margin * 2;

    // notification time stamp
    > .event-time {
      color: transparentize($fg_color, 0.5);
      @include fontsize($base_font_size - 2);
      /* HACK: the label should be baseline-aligned with a 1em label, fake this with some bottom padding */
      padding-bottom: 0.13em;

      &:ltr { text-align: right };
      &:rtl { text-align: left };
    }
  }

  // close button
  .message-close-button {
    color: lighten($fg_color, 15%);
    &:hover { color: if($variant=='light', lighten($fg_color, 30%), darken($fg_color, 10%)); }
    &:active { color: if($variant=='light', lighten($fg_color, 40%), darken($fg_color, 20%)); }
  }

  // body
  .message-body {
    color: darken($fg_color, 10%);
  }
}

// URLs in messages
.url-highlighter {
  link-color: $link_color;
}

/* Media Controls */
.message-media-control {
  padding: $base_padding * 2 1.64em; // $base_padding * 4 = 24px
  color: darken($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
  &:last-child:ltr { border-radius: 0 $base_border_radius+2 $base_border_radius+2 0; }
  &:last-child:rtl { 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;
  }
}