You can find that option through Admin Dashboard > Appearance > Customize > Blog > Single Posts (General) > Reading Progress Bar.
- Reading Progress Bar: Toggle option to enable or disable the reading progress bar.
- Bar Position: Control the position of reading progress bar (top or bottom)
- Bar Height: Control the reading progress bar height.
- Include Comments Area: The progress bar fill 100% when the last comment is shown in the browser view.
This feature has styling options as well. You can find these options through Admin Dashboard > Appearance > Customize > Blog > Single Posts (Style) > Reading Progress Bar.
- Background Color: Controls the background color of reading progress bar wrapper.
- Foreground Color: Controls the color of the foreground bar.