message: Fix mark as read button for dark theme
  This changes the mark as read button to a mask image so that it can be themed
  like other images.
  Fixes
diff: @@ -51,7 +51,7 @@ limitations under the License.
  position: absolute;
  width: 38px;
  height: 38px;
  - mask: url('$(res)/img/icon-jump-to-first-unread.svg');
  + mask-image: url('$(res)/img/icon-jump-to-first-unread.svg');
  mask-repeat: no-repeat;
  mask-position: 9px 13px;
  background: $roomtile-name-color;
  @@ -61,12 +61,20 @@ limitations under the License.
  display: block;
  width: 18px;
  height: 18px;
  - background-image: url('$(res)/img/cancel.svg');
  - background-position: center;
  - background-size: 10px;
  - background-repeat: no-repeat;
  - background-color: $primary-bg-color;
  + background: $primary-bg-color;
  border: 1.3px solid $roomtile-name-color;
  - border-radius: 99px;
  + border-radius: 10px;
  margin: 5px auto;
  }
  +
  +.mx_TopUnreadMessagesBar_markAsRead::before {
  + content: "";
  + position: absolute;
  + width: 18px;
  + height: 18px;
  + mask-image: url('$(res)/img/cancel.svg');
  + mask-repeat: no-repeat;
  + mask-size: 10px;
  + mask-position: 4px 4px;
  + background: $roomtile-name-color;
  +}
  