A recent update to Youtube removes the helpful greyed out 'Watched' badge from videos you have watched and instead provides a red progress bar at the bottom.
This style modifies the progress bar on thumbnails to make it look more like the greyed out 'watched' badge you may be accustomed to.
Using this style completely disables the progress indicator on thumbnails and instead just indicates if you have watched the video or not.
Works on main page, playlist pages, search results and sidebar. Disappears when hovered.