lbry-desktop/ui/scss/component/_file-thumbnail.scss

.file-thumbnail__viewed-bar {
  position: absolute;
  bottom: 0;
  left: 0;
  width: 100%;
  height: 5px;
  background-color: var(--color-gray-7);
}

.file-thumbnail__viewed-bar-progress {
  height: 5px;
  background-color: var(--color-primary);
}