Based on community feedback ReadCube Papers is excited to add the feature Read/Unread!
When an article is Unread in your library it will appear bold. ReadCube Papers will automatically mark the paper as "Read" after it is opened in the web viewer.
To manually mark an article as Read or Unread head to the gear icon in the right hand corner and select Read/Unread in the drop down menu.
Additionally, you can select Read/Unread in the article's metadata all the way at the bottom.